kvm: Add kvm system event crash handler

KVM kernel can send guest crash events into userspace.
Appropriate guest crash handler is called when kernel guest
crash event received. Guest crash event recognized by a
KVM_SYSTEM_EVENT_CRASH type of system event.

Signed-off-by: Andrey Smetanin <asmetanin@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Andreas Färber <afaerber@suse.de>
Message-Id: <1435924905-8926-11-git-send-email-den@openvz.org>
[Rebase: add lock/unlock iothread around qemu_system_guest_panicked - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
