webadmin: storage main-tab - use StorageSizeColumn

Storage main-tab:
Use StorageSizeColumn instead of DiskSizeColumn
since 'N/A' should be displayed when size is '0'
(similar to StorageSizeLabel in General sub-tab).

Change-Id: Ib18f6dd5f60e2421ed031e3baf79454ea7455924
Signed-off-by: Daniel Erez <derez@redhat.com>