Back up a (Flatpak-based) Gnome Boxes VM

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Generally, the Gnome Boxes docs here should work when Flatpak is not involved. But when working with the Flatpak-based Gnome Boxes, the following steps must be used to back up a VM.

# enter the flatpak's sandboxed environment
flatpak run --command=bash org.gnome.Boxes

# list the gnome boxes domain names
virsh -c qemu:///session list --all

# dump the target vm's config to xml
virsh dumpxml $VM > $VM.xml

# locate the target vm's disk image
ls ~/.var/app/org.gnome.Boxes/data/gnome-boxes/images/

# copy both the config xml and the disk image to a backup location
cp $VM.xml /some/backup/path/

cp \
	~/.var/app/org.gnome.Boxes/data/gnome-boxes/images/$VM \
    /some/backup/path/