KVM: emulator: emulate AAM

This is used by SGABIOS, KVM breaks with emulate_invalid_guest_state=1.

AAM needs the source operand to be unsigned; do the same in AAD as well
for consistency, even though it does not affect the result.

Reported-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Cc: stable@vger.kernel.org # 3.9
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>