KVM: emulator: check rex prefix inside decode_register()
KVM: emulator: check rex prefix inside decode_register()

All decode_register() callers check if instruction has rex prefix
to properly decode one byte operand. It make sense to move the check
inside.

Reviewed-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>
Signed-off-by: Gleb Natapov <03189597947f75187c4136ade35aec34feed9dc1@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs