misc/pvpanic: add shutdown event definition

misc/pvpanic: add shutdown event definition

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.

The event was added to the specification in qemu commit
73279cecca03 ("docs/specs/pvpanic: document shutdown event").

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240313-pvpanic-shutdown-header-v1-2-7f1970d66366@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>