KVM: PPC: bookehv: Add guest computation mode for irq delivery

When delivering guest IRQs, update MSR computation mode according to guest
interrupt computation mode found in EPCR.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
[agraf: remove HV dependency in the code]
Signed-off-by: Alexander Graf <agraf@suse.de>