KVM: PPC: Book3S HV: Fix software walk of guest process page tables

This fixes some bugs in the code that walks the guest's page tables.
These bugs cause MMIO emulation to fail whenever the guest is in
virtial mode (MMU on), leading to the guest hanging if it tried to
access a virtio device.

The first bug was that when reading the guest's process table, we were
using the whole of arch->process_table, not just the field that contains
the process table base address.  The second bug was that the mask used
when reading the process table entry to get the radix tree base address,
RPDB_MASK, had the wrong value.

Fixes: 9e04ba69 ("KVM: PPC: Book3S HV: Add basic infrastructure for radix guests")
Fixes: e9983344

 ("powerpc/mm/radix: Add partition table format & callback")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>