spice-file-xfer-task: unref properties on dispose

When the construction properties are set, the objects are
referenced by g_value_dup_object(), so they should be unref
once the task is done.

If the main channel stays referenced, spice session fails
to disconnect and the viewer does not close.

Signed-off-by: Jakub Janků <jjanku@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
2 jobs for master in 8 minutes and 37 seconds (queued for 25 seconds)
Status Job ID Name Coverage
  Test
passed #461002
fedora

00:08:37

passed #461003
windows

00:05:33