softmmu/runstate: add a way to detect force shutdowns

softmmu/runstate: add a way to detect force shutdowns

This can be useful for devices that might take too long to shut down
gracefully, but may have a way to shutdown quickly otherwise if needed
or explicitly requested by a force shutdown.

For now we only consider SIGTERM or the QMP quit() command a force
shutdown, since those bypass the guest entirely and are equivalent to
pulling the power plug.

Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Message-Id: <20250609212547.2859224-2-d-tatianin@yandex-team.ru>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>