KVM: PPC: e500: don't translate gfn to pfn with preemption disabled

Delay allocation of the shadow pid until we're ready to disable
preemption and write the entry.

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