qga: fix assert regression on guest-shutdown

qga: fix assert regression on guest-shutdown

Since commit 781f2b3d1e ("qga: process_event() simplification"),
send_response() is called unconditionally, but will assert when "rsp" is
NULL. This may happen with QCO_NO_SUCCESS_RESP commands, such as
"guest-shutdown".

Fixes: 781f2b3d1e5ef389b44016a897fd55e7a780bf35
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Reported-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit 844bd70b5652f30bbace89499f513e3fbbb6457a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>