KVM: x86 emulator: mask group 8 instruction as BitOp

Mask group 8 instruction as BitOp, so we can share the
code for adjust the source operand.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>