rpc: convert RPC client to use GMainLoop instead of poll

rpc: convert RPC client to use GMainLoop instead of poll

To eliminate the dependancy on GNULIB's poll impl, we need
to change the RPC client code to use GMainLoop. We don't
really want to use GIOChannel, but it provides the most
convenient way to do socket event watches with Windows
portability. The other alternative would be to use GSocket
but that is a much more complex change affecting libvirt
more broadly.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>