libqtest: add qmp(fmt, ...) -> QDict* function

Add a qtest qmp() function that returns the response object.  This
allows test cases to verify the result or to check for error responses.
It also allows waiting for QMP events.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
