KVM: x86: work around infinite loop in microcode when #AC is delivered

KVM: x86: work around infinite loop in microcode when #AC is delivered

commit 54a20552e1eae07aa240fa370a0293e006b5faed upstream.

It was found that a guest can DoS a host by triggering an infinite
stream of "alignment check" (#AC) exceptions.  This causes the
microcode to enter an infinite loop where the core never receives
another interrupt.  The host kernel panics pretty quickly due to the
effects (CVE-2015-5307).

Signed-off-by: Eric Northup <digitaleric@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[lizf: Backported to 3.4:
 - adjust filename
 - adjust context
 - add definition of AC_VECTOR]
Signed-off-by: Zefan Li <lizefan@huawei.com>