KVM: Fix cpuid leaf 0xb loop termination

For cpuid leaf 0xb the bits 8-15 in ECX register define the end of counting
leaf.      The previous code was using bits 0-7 for this purpose, which is
a bug.

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>