gio-tool: Hold GVolumeMonitor reference during operations

Releasing GVolumeMonitor before g_volume_mount finish cause that
g_volume_get_mount returns NULL, because the mount is not correctly
propagated to the volume.

(Backported from commit 88b8ebb5 with
minor merge conflicts.)

GNOME/glib#1458
1 job for cherry-pick-volume-mount-2-56 in 2 minutes and 21 seconds (queued for 70 minutes and 10 seconds)
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