KVM: PPC: Use KVM_BOOK3S_HANDLER

So far we had a lot of conditional code on CONFIG_KVM_BOOK3S_64_HANDLER.
As we're moving towards common code between 32 and 64 bits, most of
these ifdefs can be moved to a more generic term define, called
CONFIG_KVM_BOOK3S_HANDLER.

This patch adds the new generic config option and moves ifdefs over.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>