libxl: Reject VM config referencing nwfilters

libxl: Reject VM config referencing nwfilters

The Xen libxl driver does not support nwfilter. Introduce a
deviceValidateCallback function with a check for nwfilters, returning
VIR_ERR_CONFIG_UNSUPPORTED if any are found. Also fail to start any
existing VMs referencing nwfilters.

Drivers generally ignore unrecognized XML configuration, but ignoring
a user's request to filter VM network traffic can be viewed as a
security issue.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>