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 <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

fabianfreyer/libvirt,jfehlig/libvirt,andreabolognani/libvirt,zippy2/libvirt,zippy2/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,eskultety/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,libvirt/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,andreabolognani/libvirt,eskultety/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt,andreabolognani/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt