pc: implement pc-dimm device abstraction

pc: implement pc-dimm device abstraction

Each hotplug-able memory slot is a PCDIMMDevice.
A hot-add operation for a memory device:
- creates a new PCDIMMDevice and makes hotplug controller to map it into
  guest address space

Hotplug operations are done through normal device_add commands.
For migration case, all hotplugged memory devices on source should be
specified on target's command line using '-device' option with
properties set to the same values as on source.

To simplify review, patch introduces only PCDIMMDevice QOM skeleton that
will be extended by following patches to implement actual memory hotplug
and related functions.

Signed-off-by: Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>