Add a way to return FDs to the RPC caller

Add a way to return FDs to the RPC caller

Currently the libvirt client can pass FDs to the server, but the
dispatch mechanism provides no way to return FDs back from the
server to the client. Tweak the dispatch code, such that if a
dispatcher returns '1', this indicates that it populated the
virNetMessagePtr with FDs to return

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