KVM: PPC: bookehv64: Add support for interrupt handling

Add interrupt handling support for 64-bit bookehv hosts. Unify 32 and 64 bit
implementations using a common stack layout and a common execution flow starting
from kvm_handler_common macro. Update documentation for 64-bit input register
values. This patch only address the bolted TLB miss exception handlers version.

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