Let qmp_dispatch() copy the 'id' field. That way any qmp client will
conform to the specification, including QGA. Furthermore, it
simplifies the work for qemu monitor.

CC: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>