KVM: x86 emulator: fix 'mov rm,sreg' instruction decoding

The source operand of 'mov rm,sreg' is segment register, not
general-purpose register, so remove SrcReg from decoding.

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