KVM: x86 emulator: Add LOCK prefix validity checking

Instructions which are not allowed to have LOCK prefix should
generate #UD if one is used.

[avi: fold opcode 82 fix from another patch]

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>