iotests: fortify compare_images() against crashes

iotests: fortify compare_images() against crashes

Fortify compare_images() to be more discerning about the status codes it
receives. If qemu_img() returns an exit code that implies it didn't
actually perform the comparison, treat that as an exceptional
circumstance and force the caller to be aware of the peril.

If a negative test is desired (perhaps to test how qemu_img compare
behaves on malformed images, for instance), it is still possible to
catch the exception in the test and deal with that circumstance
manually.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220321201618.903471-6-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>