gio: Complete also activation roots of volumes
gio: Complete also activation roots of volumes

Currently, "gio mount google-drive<tab>" isn't completed even though
that volume exists for google-drive://oholy@redhat.com/. Let's use
"gio mount -li" output to complete also activation roots of volumes.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib