Handle bit 1 write, then post event to monitor.

Suggested by Paolo, declear a new event, using GUEST_PANICKED could
cause upper layers to react by shutting down or rebooting the guest.

In advance for extention, add GuestPanicInformation in event message.

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20200114023102.612548-3-pizhenwei@bytedance.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>