webadmin: Restricted VNIC names to ASCII characters

Used to allow Unicode characters as well, which meant it was more
permissive than the backend validation.

Change-Id: Id85f0cec55ab073eff0a26acc884350d0977a1a0
Bug-Url: https://bugzilla.redhat.com/1002146
Signed-off-by: Lior Vernia <lvernia@redhat.com>