One of the tests checks the ability to boot multiple guests at once, and
it is using the same (empty) disk image for all. Modern QEMU rejects
this sharing, but it is harmless in context of this test case, so we
mark the disk as shareable to allow it.
Reviewed-by: Laine Stump <email@example.com>
Signed-off-by: Daniel P. Berrangé <firstname.lastname@example.org>