nodedev: Add parser validation for node devices

nodedev: Add parser validation for node devices

At the moment, this is only for mediated devices. When a new mediated
device is created or defined, the xml is expected specify the nodedev
name of an existing device as its parent. We were not previously
validating this and were simply accepting any string here.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>