KVM: x86 emulator: Use opcode::execute for INS/OUTS from/to port in DX

INSB       : 6C
INSW/INSD  : 6D
OUTSB      : 6E
OUTSW/OUTSD: 6F

The I/O port address is read from the DX register when we decode the
operand because we see the SrcDX/DstDX flag is set.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>