KVM: PPC: Add SR swapping code

Later in this series we will move the current segment switch code to
generic code and make that call hooks for the specific sub-archs (32
vs. 64 bit). This is the hook for 32 bits.

It enabled the entry and exit code to swap segment registers with
values from the shadow cpu structure.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>