KVM: PPC: Book3S HV: Handle guest using doorbells for IPIs

* SRR1 wake reason field for system reset interrupt on wakeup from nap
  is now a 4-bit field on P8, compared to 3 bits on P7.

* Set PECEDP in LPCR when napping because of H_CEDE so guest doorbells
  will wake us up.

* Waking up from nap because of a guest doorbell interrupt is not a
  reason to exit the guest.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>