KVM: SVM: Set the 'g' bit of the cs selector for cross-vendor migration

The hardware does not set the 'g' bit of the cs selector and this breaks
migration from amd hosts to intel hosts. Set this bit if the segment
limit is beyond 1 MB.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>