KVM: Hoist SVM's get_cs_db_l_bits into core code.

SVM gets the DB and L bits for the cs by decoding the segment.  This
is in fact the completely generic code, so hoist it for kvm-lite to use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@qumranet.com>