engine: Improve message when network is mis-configured

The patch adds a specific message to the host general sub tab
for the case of a VM network isn't implemented so on the host.

In addition, it adjusts the indicator of the same error in the
event log to warning so it could be noticeable easily.

Change-Id: I0f51fdc0d66cc55020858391dabd026b88e4fe35
Bug-Url: https://bugzilla.redhat.com/951920
Signed-off-by: Moti Asayag <masayag@redhat.com>