docs/specs/pvpanic: document shutdown event

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.

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>
Message-Id: <20240310-pvpanic-shutdown-spec-v1-1-b258e182ce55@t-8ch.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/pvpanic.rst

@@ -29,6 +29,8 @@ bit 1
   a guest panic has happened and will be handled by the guest;
   the host should record it or report it, but should not affect
   the execution of the guest.
+bit 2
+  a regular guest shutdown has happened and should be processed by the host
 
 PCI Interface
 -------------