engine: add vnic displays <UNKNOWN> in the event instead of interface name

InterfaceName is defined as CustomLogField annotation.
Properties defined as CustomLogField should have public getter,
in this case the getter was protected.

Change-Id: I5824e32340403991b274c606fbe4334297ac851e
Bug-Url: https://bugzilla.redhat.com/907154
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>