network: Implement virConnectSupportsFeature()

network: Implement virConnectSupportsFeature()

So far, it was not needed, but shortly a client will want to know
whether virNetworkUpdate() API is fixed or not. See next commits
for more info.

Side note, this driver's implementation is called only when using
sub-driver's connection, i.e. "network:///system". For any other
URI the corresponding hypervisor's driver callback is called.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>