KVM: x86 emulator: fix Src2CL decode

Src2CL decode (used for double width shifts) erronously decodes only bit 3
of %rcx, instead of bits 7:0.

Fix by decoding %cl in its entirety.

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