KVM: x86 emulator: qualify OpReg inhibit_byte_regs hack

OpReg decoding has a hack that inhibits byte registers for movsx and movzx
instructions.  It should be replaced by something better, but meanwhile,
qualify that the hack is only active for the destination operand.

Note these instructions only use OpReg for the destination, but better to
be explicit about it.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>