rpc: move state stop into virNetDaemon class

rpc: move state stop into virNetDaemon class

Currently the remote daemon code is responsible for calling virStateStop
in a background thread. The virNetDaemon code wants to synchronize with
this during shutdown, however, so the virThreadPtr must be passed over.

Even the limited synchronization done currently, however, is flawed and
to fix this requires the virNetDaemon code to be responsible for calling
virStateStop in a thread more directly.

Thus the logic is moved over into virStateStop via a further callback
to be registered by the remote daemon.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>