libxl: use preferred syntax for network device creation with upstream qemu

libxl: use preferred syntax for network device creation with upstream qemu

Markus Armbruster points out in <m3r582pzc1.fsf@blackfin.pond.sub.org>
on qemu-devel that this is the prefered syntax going forward. Using it avoid
needlessly instantiating a qemu "vlan" and instead creates a simply host end
point and device.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>