Comment 4 for bug 1612875

Yeah apparently this doesn't exist in libvirt 1.2.1:

https://github.com/libvirt/libvirt/blob/v1.2.1/src/conf/nwfilter_conf.c#L2979

Which is our minimum supported version, and ubuntu 14.04 runs with 1.2.2.

In ubuntu 16.04 we're running with libvirt 1.3.1 which shows the check and error:

https://github.com/libvirt/libvirt/blob/v1.3.1/src/conf/nwfilter_conf.c#L3099-L3101

            virReportError(VIR_ERR_OPERATION_FAILED,
                           _("filter '%s' already exists with uuid %s"),
                           def->name, uuidstr);

It's unfortunate that there isn't a specific error code, we'll have to check for VIR_ERR_OPERATION_FAILED and the message.