conf: expand network device callbacks to cover resolving NIC type

conf: expand network device callbacks to cover resolving NIC type

Currently the QEMU driver will call directly into the network driver
impl to modify resolve the atual type of NICs with type=network. It
has todo this before it has allocated the actual NIC. This introduces
a callback system to allow us to decouple the QEMU driver from the
network driver.

This is a short term step, as it ought to be possible to achieve the
same end goal by simply querying XML via the public network API. The
QEMU code in question though, has no virConnectPtr conveniently
available at this time.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>