If the device is disconnected, we should also disconnect the modem; and
while we disconnect the device we should clean the connection context.
Otherwise the modem will surprise us by emitting PREPARED signal when
the device is no longer PREPARED:

  NetworkManager[28469]: <info>  [1462383185.8714] (ttyACM1): modem state changed, 'disconnecting' --> 'registered' (reason: user-requested)
  NetworkManager[28469]: <info>  [1462383185.8715] (ttyACM1): modem state changed, 'registered' --> 'connecting' (reason: user-requested)
  NetworkManager[28469]: <info>  [1462383185.8716] device (ttyACM1): state change: deactivating -> disconnected (reason 'connection-removed') [110 30 38]
  NetworkManager[28469]: <info>  [1462383185.8759] (ttyACM1): modem state changed, 'connecting' --> 'disconnecting' (reason: user-requested)
  NetworkManager[28469]: <warn>  [1462383185.8937] (ttyACM1): failed to connect modem: Dial operation has been cancelled

  (NetworkManager:28469): NetworkManager-wwan-CRITICAL **: modem_prepare_result: assertion 'state == NM_DEVICE_STATE_PREPARE' failed

  Program received signal SIGTRAP, Trace/breakpoint trap.
  g_logv (log_domain=0x7fffea31bc47 "NetworkManager-wwan", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7fffffffcfc0) at gmessages.c:1086
  1086              g_private_set (&g_log_depth, GUINT_TO_POINTER (depth));
  (gdb) bt
  #0  0x00007ffff4ebe643 in g_logv (log_domain=0x7fffea31bc47 "NetworkManager-wwan", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7fffffffcfc0) at gmessages.c:1086
  #1  0x00007ffff4ebe7bf in g_log (log_domain=<optimized out>, log_level=<optimized out>, format=<optimized out>) at gmessages.c:1119
  Python Exception <type 'exceptions.RuntimeError'> Cannot locate object file for block.:
  #2  0x00007ffff2ce6dac in ffi_call_unix64#3  0x00007ffff2ce66d5 in ffi_call (cif=cif@entry=0x7fffffffd300, fn=<optimized out>, rvalue=0x7fffffffd230, avalue=avalue@entry=0x7fffffffd1d0) at ../src/x86/ffi64.c:522
  #4  0x00007ffff51b55a5 in g_cclosure_marshal_generic_va (closure=0x555555b30cb0, return_value=0x0, instance=0x555555a8d360, args_list=<optimized out>, marshal_data=0x0, n_params=2, param_types=0x555555c2bb60) at gclosure.c:1600
  #5  0x00007ffff51b4b37 in _g_closure_invoke_va (closure=closure@entry=0x555555b30cb0, return_value=return_value@entry=0x0, instance=instance@entry=0x555555a8d360, args=args@entry=0x7fffffffd5b8, n_params=2, param_types=0x555555c2bb60) at gclosure.c:864
  #6  0x00007ffff51ce117 in g_signal_emit_valist (instance=instance@entry=0x555555a8d360, signal_id=signal_id@entry=168, detail=detail@entry=0, var_args=var_args@entry=0x7fffffffd5b8) at gsignal.c:3292
  #7  0x00007ffff51cf2e8 in g_signal_emit_by_name (instance=instance@entry=0x555555a8d360, detailed_signal=detailed_signal@entry=0x7fffea074cdd "prepare-result") at gsignal.c:3479
  #8  0x00007fffea011fd3 in connect_context_step (self=self@entry=0x555555a8d360 [NMModemBroadband]) at nm-modem-broadband.c:529
  #9  0x00007fffea01264d in connect_ready (simple_iface=<optimized out>, res=<optimized out>, self=0x555555a8d360 [NMModemBroadband]) at nm-modem-broadband.c:378
  #10 0x00007ffff546a297 in g_simple_async_result_complete (simple=0x7fffe00104e0 [GSimpleAsyncResult]) at gsimpleasyncresult.c:801
  #11 0x00007fffe9d82fec in connect_context_complete_and_free (ctx=ctx@entry=0x555555c52f60) at mm-modem-simple.c:93
  #12 0x00007fffe9d83155 in simple_connect_ready (self=0x7fffdc00b9f0 [MMModemSimple], res=0x7fffdc004410, ctx=0x555555c52f60) at mm-modem-simple.c:159
  #13 0x00007ffff547af93 in g_task_return_now (task=0x7fffdc004410 [GTask]) at gtask.c:1106
  #14 0x00007ffff547b62e in g_task_return (task=0x7fffdc004410 [GTask], type=<optimized out>) at gtask.c:1164
  #15 0x00007ffff54d4239 in reply_cb (connection=<optimized out>, res=<optimized out>, user_data=0x7fffdc004410) at gdbusproxy.c:2570
  #16 0x00007ffff547af93 in g_task_return_now (task=0x7fffdc004340 [GTask]) at gtask.c:1106
  #17 0x00007ffff547b62e in g_task_return (task=0x7fffdc004340 [GTask], type=<optimized out>) at gtask.c:1164
  #18 0x00007ffff54c8c9f in g_dbus_connection_call_done (source=<optimized out>, result=0x555555a60920, user_data=0x7fffdc004340) at gdbusconnection.c:5702
  #19 0x00007ffff547af93 in g_task_return_now (task=0x555555a60920 [GTask]) at gtask.c:1106
  #20 0x00007ffff547afc9 in complete_in_idle_cb (task=0x555555a60920) at gtask.c:1120
  #21 0x00007ffff4eb7d7a in g_main_context_dispatch (context=0x555555a4a000) at gmain.c:3152
  #22 0x00007ffff4eb7d7a in g_main_context_dispatch (context=context@entry=0x555555a4a000) at gmain.c:3767
  #23 0x00007ffff4eb80b8 in g_main_context_iterate (context=0x555555a4a000, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3838
  #24 0x00007ffff4eb838a in g_main_loop_run (loop=0x555555a48780) at gmain.c:4032
  #25 0x00005555555aebf2 in main (argc=1, argv=0x7fffffffdc48) at main.c:477
  (gdb)