KVM: Enable ERMS feature support for KVM

This patch exposes ERMS feature to KVM guests.

The REP MOVSB/STOSB instruction can enhance fast strings attempts to
move as much of the data with larger size load/stores as possible.

Signed-off-by: Yang, Wei <wei.y.yang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>