cpu: Add crash_occurred flag into CPUState

CPUState::crash_occurred field inside CPUState marks
that guest crash occurred. This value is added into
cpu common migration subsection.

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: <1435924905-8926-12-git-send-email-den@openvz.org>
[Document the new field. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
