engine_page: add exception for ENETUNREACH

Due the change to use vdscli (xmlrpc) to communicate with VDSM
to verify the networks available, it can cause an exception of
Network is unreachable if there is no (ovirtmgmt) interface to
communicate with vdsm. This patch handle the exception.

Change-Id: I77db22b372820968a5af86d65fa1c40574a7ebd1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1104009
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>