webadmin: disks sub-tab - fix columns creation

Following sorting support, disks columns instantiation
should be ensured to happen once. Otherwise, the columns get
duplicated on type navigation (radio-buttons of all/images/directlun).

Hence, applied the same solution introduced in [1] on MainTabDiskView
to disks sub-tab as well.

[1] http://gerrit.ovirt.org/29375/

Change-Id: Idc774e26c9a3d954e9d7b8a4fee9206358269926
Signed-off-by: Daniel Erez <derez@redhat.com>