Fix real mode guest migration

Older KVM versions save CS dpl value to an invalid value for real mode guests
(0x3). This patch detect this situation when loading CPU state and set all the
segments dpl to zero.
This will allow migration from older KVM on host without unrestricted guest
to hosts with restricted guest support.
For example migration from a Penryn host (with kernel 2.6.32) to
a Westmere host (for real mode guest) will fail with "kvm: unhandled exit 80000021".

Signed-off-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
