KVM: PPC: Book3S HV: Page table construction and page faults for radix guests

KVM: PPC: Book3S HV: Page table construction and page faults for radix guests

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1675806
This adds the code to construct the second-level ("partition-scoped" in
architecturese) page tables for guests using the radix MMU.  Apart from
the PGD level, which is allocated when the guest is created, the rest
of the tree is all constructed in response to hypervisor page faults.

As well as hypervisor page faults for missing pages, we also get faults
for reference/change (RC) bits needing to be set, as well as various
other error conditions.  For now, we only set the R or C bit in the
guest page table if the same bit is set in the host PTE for the
backing page.

This code can take advantage of the guest being backed with either
transparent or ordinary 2MB huge pages, and insert 2MB page entries
into the guest page tables.  There is no support for 1GB huge pages
yet.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 5a319350a46572d073042a3194676099dd2c135d)
Signed-off-by: Breno Leitao <breno.leitao@gmail.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>