conf: Validate device on update-device

conf: Validate device on update-device

https://bugzilla.redhat.com/show_bug.cgi?id=1439991

Whenever a device is being updated via
virDomainUpdateDeviceFlags() API, we parse the device XML and
ideally run some generic checks to validate the configuration
(e.g. if device defines per-device boot order but the domain has
os/boot element already). Well, that's the theory - due to a
missing check we've jumped early from that check function.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>