KVM: PPC: Book3S PR: Use 64k host pages where possible

Currently, PR KVM uses 4k pages for the host-side mappings of guest
memory, regardless of the host page size.  When the host page size is
64kB, we might as well use 64k host page mappings for guest mappings
of 64kB and larger pages and for guest real-mode mappings.  However,
the magic page has to remain a 4k page.

To implement this, we first add another flag bit to the guest VSID
values we use, to indicate that this segment is one where host pages
should be mapped using 64k pages.  For segments with this bit set
we set the bits in the shadow SLB entry to indicate a 64k base page
size.  When faulting in host HPTEs for this segment, we make them
64k HPTEs instead of 4k.  We record the pagesize in struct hpte_cache
for use when invalidating the HPTE.

For now we restrict the segment containing the magic page (if any) to
4k pages.  It should be possible to lift this restriction in future
by ensuring that the magic 4k page is appropriately positioned within
a host 64k page.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>