sdl: Fix termination in -no-shutdown mode

Just like the monitor does, we need to clear no_shutdown before calling
qemu_system_shutdown_request on quit requests. Otherwise, QEMU just
stops the VM.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>