domain_conf: Validate redirdev after parsing

domain_conf: Validate redirdev after parsing

There's currently just one limitation: redirdevs that want to go
on USB bus require a USB controller, surprisingly.
At the same time, since I'm using virDomainDefHasUSB() in this
new validator function, it has to be moved a few lines up and
also its header needed to be changed a bit: it is now taking a
const pointer to domain def since it's not changing anything in
there.

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