engine: Add audit log for network without interfaces

An audit log should be added to the use case in which
vdsm doesn't report any interface to a reported network.
This error was hidden by the event log of 'network is
attached to multiple interfaces'.

Change-Id: I3d2e9a3793ac1511debc25d69755ed0e99598cee
Bug-Url: https://bugzilla.redhat.com/881055
Signed-off-by: Moti Asayag <masayag@redhat.com>