KVM: x86 emulator: add decoding of X,Y parameters from Intel SDM

Add decoding of X,Y parameters from Intel SDM which are used by string
instruction to specify source and destination. Use this new decoding
to implement movs, cmps, stos, lods in a generic way.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>