qapi event: convert GUEST_PANICKED

'monitor.h' is still included in target-s390x/kvm.c, since I have
no good way to verify whether other code need it on my x86 host.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>