conf: Move virDomainDeviceAddressIsValid() to device_conf

conf: Move virDomainDeviceAddressIsValid() to device_conf

The function is called on a virDomainDeviceInfo, so it
should be declared along with it.

Moving this function requires moving and making public
virDomainDeviceCCWAddressIsValid() as well, but that's
perfectly fine since the same reasoning above also
applies to it, due to virDomainDeviceCCWAddress being
(correctly) declared in device_conf.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>