memory-device: add and use memory_device_get_region_size()

memory-device: add and use memory_device_get_region_size()

We will factor out get_memory_region() from pc-dimm to memory device code
soon. Once that is done, get_region_size() can be implemented
generically and essentially be replaced by
memory_device_get_region_size (and work only on get_memory_region()).

We have some users of get_memory_region() (spapr and pc-dimm code) that are
only interested in the size. So let's rework them to use
memory_device_get_region_size() first, then we can factor out
get_memory_region() and eventually remove get_region_size() without
touching the same code multiple times.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181005092024.14344-10-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>