qemu: stop passing in virConnectPtr for looking up networks

qemu: stop passing in virConnectPtr for looking up networks

When setting up graphics, we sometimes need to resolve networks,
requiring the caller to pass in a virConnectPtr, except sometimes they
pass in NULL. Use virGetConnectNetwork() to acquire the connection to
the network driver when it is needed.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>