nwfilter: allow opening with nwfilter:///system URI

nwfilter: allow opening with nwfilter:///system URI

Allow the possibility of opening a connection to only the storage
driver, by defining a nwfilter:///system URI and registering a fake
hypervisor driver that supports it.

The hypervisor drivers can now directly open a nwfilter driver
connection at time of need, instead of having to pass around a
virConnectPtr through many functions. This will facilitate the later
change to support separate daemons for each driver.

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