webadmin: Set uniform width to vNIC entries

Added a set width for the vNIC entries in the add/edit VM dialog, to
avoid longer vNIC names indenting the other UI fields.

Change-Id: If0695a84aaee747b7020961da3392a60507b2fe6
Bug-Url: https://bugzilla.redhat.com/1023035
Signed-off-by: Lior Vernia <lvernia@redhat.com>