KVM: PPC: Add helpers for CR, XER

We now have helpers for the GPRs, so let's also add some for CR and XER.

Having them in the PACA simplifies code a lot, as we don't need to care
about where to store CC or not to overflow any integers.

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