hw/misc/pvpanic: add support for normal shutdowns

hw/misc/pvpanic: add support for normal shutdowns

Shutdown requests are normally hardware dependent.
By extending pvpanic to also handle shutdown requests, guests can
submit such requests with an easily implementable and cross-platform
mechanism.

Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>
Message-Id: <20240527-pvpanic-shutdown-v8-5-5a28ec02558b@t-8ch.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>