webadmin: Provide confirmation prompt while unplugging a Vnic

When the user clicks "ok" on edit vnic dialog-
1. If the user changed the vnic to be unplugged a confrimation window
should be shown.
2. Otherwise, the dialog should be saved.

Change-Id: Iaf1c384f0205bb8b30cda038963eb49eba1dae55
Bug-Url: https://bugzilla.redhat.com/859024
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>