KVM: PPC: booke: check for signals in kvmppc_vcpu_run

Currently we check prior to returning from a lightweight exit,
but not prior to initial entry.

book3s already does a similar test.

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