[BUG] libxenstat does parse networks with openvswitch only bridges

[Richard Kojedzinszky <richard@xxxxxxxxx>]

Dear xen developers,

When xen is deployed in a way that no linux bridges exists on the host, 
then 
https://github.com/xen-project/xen/blob/stable-4.14/tools/xenstat/libxenstat/src/xenstat_linux.c#L313 
will get devBridge as empty string, and then conditions on 
https://github.com/xen-project/xen/blob/stable-4.14/tools/xenstat/libxenstat/src/xenstat_linux.c#L326 
will evaluate to true, effectively discarding any parsed results. As a 
workaround, just creating a fake unused bridge will resolve the issue, 
howewer libxsenstat itself should take care of such cases.
The same is true for later branches as well.

Regards,
Richard Kojedzinszky