target-ppc/kvm: Add a wrapper function to check for KVM-PR

It makes more sense if we have a proper function to check
for KVM-PR than to check for the GET_PVINFO extension all
over the place.

Signed-off-by: Thomas Huth <thuth@redhat.com>
[dwg: Expanded a comment to discourage overuse of this function]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
