webadmin: storage-general sub-tab use StorageSizeLabel

Storage main-tab -> General sub-tab:
Use StorageSizeLabel instead of DiskSizeLabel since 'N/A' should be
displayed when size is '0' (only in general sub-tab).

Change-Id: I1c1722a1a745d3a91bb87fc5c28dd8a6fb1106ab
Signed-off-by: Daniel Erez <derez@redhat.com>