KVM: PPC: Add generic segment switching code

KVM: PPC: Add generic segment switching code

This is the code that will later be used instead of book3s_64_slb.S. It
does the last step of guest entry and the first generic steps of guest
exiting, once we have determined the interrupt is a KVM interrupt.

It also reads the last used instruction from the guest virtual address
space if necessary, to speed up that path.

The new thing about this file is that it makes use of generic long load
and store functions and calls a macro to fill in the actual segment
switching code. That still needs to be done differently for book3s_32 and
book3s_64.

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