KVM: PPC64: booke: Set interrupt computation mode for 64-bit host

64-bit host needs to remain in 64-bit mode when an exception take place.
Set interrupt computaion mode in EPCR register.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>