qemu_hotplug: Do not allow absent values in rom settings

qemu_hotplug: Do not allow absent values in rom settings

If there are absent values in an already existing element
specifying rom settings, we simply use the old ones. This
behaviour is not desired, as users might think that deleting the
element from XML would delete the setting (because the hotplug
succeeds) - which does not happen. Because of that, we should not
accept an interface without elements that cannot be changed.

Therefore, we should not allow absent values for already existing
rom setting during hotplug.

Resolves: https://issues.redhat.com/browse/RHEL-7109
Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>