build: switch over to new rpc generator code

build: switch over to new rpc generator code

This replaces use of 'rpcgen' with our new python impl of
the RPC code generator. Since the new impl generates code
that matches our style/coding rules, and does not contain
long standing bugs, we no longer need to post-process the
output.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>