KVM: PPC: Book3s: PR: Disable preemption in vcpu_run

When entering the guest, we want to make sure we're not getting preempted
away, so let's disable preemption on entry, but enable it again while handling
guest exits.

Reported-by: Jörg Sommer <joerg@alea.gnuu.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>