qemu: snapshot: Mark file becoming backingStore as read-only

For any backing file we set 'read-only' to true, but didn't do this when
modifying the recorded backing store when creating external snapshots.

This meant that qemu would attempt to open the backing-file read-write.
This would fail for example when selinux is used as qemu doesn't have
write permission for the backing file.

https://bugzilla.redhat.com/show_bug.cgi?id=1781079

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>