pvpanic: Emit GUEST_PVSHUTDOWN QMP event on pvpanic shutdown signal

pvpanic: Emit GUEST_PVSHUTDOWN QMP event on pvpanic shutdown signal

Emit a QMP event on receiving a PVPANIC_SHUTDOWN event. Even though a typical
SHUTDOWN event will be sent, it will be indistinguishable from a shutdown
originating from other cases (e.g. KVM exit due to KVM_SYSTEM_EVENT_SHUTDOWN)
that also issue the guest-shutdown cause.
A management layer application can detect the new GUEST_PVSHUTDOWN event to
determine if the guest is using the pvpanic interface to request shutdowns.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Message-Id: <20240527-pvpanic-shutdown-v8-6-5a28ec02558b@t-8ch.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>