rpc: pass virNetServer to post-exec restart callback in typesafe manner

rpc: pass virNetServer to post-exec restart callback in typesafe manner

The virNetServer class is passing a pointer to itself to the
virNetServerClient as a 'void *' pointer. This is presumably due to fact
that the virnetserverclient.h file doesn't see the virNetServerPtr
typedef. The typedef is easily movable though, which lets us get
typesafe parameter passing, removing the confusion of passing two
distinct 'void *' pointers to one method.

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