ppc: Fix conditions for delivering external interrupts to a guest

External interrupts can bypass the MSR_EE test if they occur in guest
mode and LPES0 is clear. In that case they are directed to the hypervisor

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
