core: Validate vm nic name has no special characters

The patch validates the vm nic name is consisted of
a-zA-Z0-9-_. only. This is verified on the UI but
should be enforced when adding nics via the API.

Change-Id: I596f546473b1e4c8832aa56dd56320df514977ae
Bug-Url: https://bugzilla.redhat.com/973971
Signed-off-by: Moti Asayag <masayag@redhat.com>