WIP: Fix races with cancelling QMI connections repeatedly

Hi,

this patch set fixes a couple of things that get wrong with a simple test case. Needs a QMI modem.

First make sure there are some connects interposed with disconnections, so that we'll eventually loose track in an unfinished/cancelled connection attempt.

# for i in $(seq 0 9); do
>     mmcli --modem 0 --simple-connect=apn=,ip-type=ipv4 &
>     mmcli --modem 0 --simple-disconnect &
>     sleep 0.2
> done

Then let things finish and try to walk the disconnection path again.

# sleep 2
# mmcli --modem 0 --simple-disconnect

ModemManager will eventually either crash, or will be unable to connect or disconnect.