webadmin: Fix QoS empty selection bug

When selecting empty QoS in the network dialog, the selection changed
automatically to the first value in the list.

The bug was fixed. Added a new default value [Unlimited].

Change-Id: I056f3291298426ed71fd75052fc5bc977b1591e5
Signed-off-by: Ofri Masad <omasad@redhat.com>