webadmin: Unified VNIC update logic on VM creation

Towards adding the ability to add and remove VNICs in the add/edit VM
dialogs, the VNIC logic is unified so it is the same for all kinds of
VM creation flows (e.g. from scratch, from template...); VNICs may
always be added, edited or removed.

Change-Id: Ifbd61031572a3462af46f24c64fb5bb80343f086
Bug-Url: https://bugzilla.redhat.com/1003779
Signed-off-by: Lior Vernia <lvernia@redhat.com>