Add fields to PACA

For KVM we need to store some information in the PACA, so we
need to extend it.

This patch adds KVM SLB shadow related entries to the PACA and
a field that indicates if we're inside a guest.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>