rpc: refactor way connection object is generated for remote dispatch

rpc: refactor way connection object is generated for remote dispatch

Calling a push_privconn method to directly push the connection object
name into the arg list is inconvenient. Refactor so that we acquire
the connection variable name upfront, and push it to the arg list
separately. This allows various hardcoded usage of "priv->conn" to
be parameterized.

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