This change adds a new attribute - physical_disks - to
dracclient.resources.raid.VirtualDisk objects. The attribute is a
list of IDs of the physical disks that a virtual disk is composed of.
This allows clients of the library to obtain a more complete view of
the current RAID configuration.
Change-Id: If4f762d3bc115a971d0392af8270f5440ef43913
Closes-bug: #1651702