webadmin: Added host interface label character validation

Previously the characters input into a newly-assigned label were only
validated by the backend, now the dialog checks that no illegal
characters were entered as well.

Change-Id: I25bef382cb658e6e78bf5e2e0cfd1a273280d953
Bug-Url: https://bugzilla.redhat.com/1063846
Signed-off-by: Lior Vernia <lvernia@redhat.com>