CPUPhysMemoryClient: Pass guest physical address not region offset

CPUPhysMemoryClient: Pass guest physical address not region offset

When we're trying to get a newly registered phys memory client updated
with the current page mappings, we end up passing the region offset
(a ram_addr_t) as the start address rather than the actual guest
physical memory address (target_phys_addr_t).  If your guest has less
than 3.5G of memory, these are coincidentally the same thing.  If
there's more, the region offset for the memory above 4G starts over
at 0, so the set_memory client will overwrite it's lower memory entries.

Instead, keep track of the guest phsyical address as we're walking the
tables and pass that to the set_memory client.

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