In order for users to find a volume that they have attached to
a vm, it is valuable to be able to find it in a consistent
location. A following patch wil accomplish this by setting
the serial number of the device to the uuid of the volume.
This patch prepares for that change by allowing serial numbers
to be set in the libvirt config disk object.
Prepares to fix bug 1004328
Change-Id: Iecdfc17b45e1c38df50f844f127c0e95558ab22c
6cb3520587