webadmin: truncated text in VMs-Disks sub-tab

Fixed truncated headers VMs -> Disks sub-tab.

Change-Id: I2e9c4853bdd6da18801be7c04a0c201331afb7a4
Bug-Url: https://bugzilla.redhat.com/952457
Signed-off-by: Daniel Erez <derez@redhat.com>