afc: Fix force unmount of devices

When unplugging an afc device without unmounting it first, the
corresponding gvfsd-afc process does not go away in spite of the
exit(1) workaround added for bgo#708288.

This happens because idevice_event_unsubscribe() in called from the
thread where the _idevice_event_cb() runs in. libimobiledevice (through
libusbmuxd) creates a thread to watch for device events, and calls the
idevice_event_cb_t from that thread. idevice_event_unsubscribe() then
forcfully kills that thread (see usbmuxd_unsubscribe() in libusbmuxd),
so exit(1) code after the call to idevice_event_unsubscribe() never gets
a chance to run.

Moving the call to idevice_event_unsubscribe() and the
g_vfs_backend_force_unmount() call to run in the main loop through
g_idle_add() avoids this issue, and allows the code handling force
unmounts to run as expected.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=708288