qemuDomainObjStart: Warn on corrupted image

qemuDomainObjStart: Warn on corrupted image

If the managedsave image is corrupted, e.g. the XML part is, we fail to
parse it and throw an error, e.g.:

error: Failed to start domain jms8
error: XML error: missing security model when using multiple labels

This is okay, as we can't really start the machine and avoid undefined
qemu behaviour. On the other hand, the error message doesn't give a
clue to users what should they do. The consensus here would be to thrown
a warning to logs saying "Hey, you've got a corrupted file".

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>