CPUPhysMemoryClient: Fix typo in phys memory client registration

CPUPhysMemoryClient: Fix typo in phys memory client registration

When we register a physical memory client, we try to walk the page
tables, calling the set_memory hook for every entry.  Effectively
playing catchup for the client for everything already registered.
With this type, we only walk the 2nd entry of the l1 table,
typically missing all of the registered memory.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>