report guest crash information in GUEST_PANICKED event

it's not very convenient to use the crash-information property interface,
so provide a CPU class callback to get the guest crash information, and pass
that information in the event

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Message-Id: <1487053524-18674-3-git-send-email-den@openvz.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
