KVM: x86: Return error on cmpxchg16b emulation

cmpxchg16b is currently unimplemented in the emulator. The least we can do is
return error upon the emulation of this instruction.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>