memory-device: factor out get_memory_region() from pc-dimm

The memory region is necessary for plugging/unplugging a memory device.
The region size (via get_region_size()) is no longer sufficient, as
besides the alignment, also the region itself is required in order to
add it to the device memory region of the machine via
- memory_region_add_subregion
- memory_region_del_subregion

So, to factor out plugging/unplugging of memory devices from pc-dimm
code, we have to factor out access to the memory region first.

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: <20181005092024.14344-11-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>