pc-dimm: use same mechanism for [get|set]_addr

[get|set]_addr are two counterpart to access PCDIMMDevice.addr.

Since we have already set up a property PC_DIMM_ADDR_PROP for this
field and use this mechanism in set_addr, it would be more proper to use
the same mechanism in get_addr.

This patch uses object_property_get_uint() to replace the direct memory
access to make [get|set]_addr with the same mechanism.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Message-Id: <20190211064629.20186-1-richardw.yang@linux.intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>