libxl: Allow network driver domains when run_hotplug_scritps is set

libxl: Allow network driver domains when run_hotplug_scritps is set

As of commit 05bfd984dfe7014f1f5ea1133608b9bab589c120, hotplug scripts
are not run if backend_domid != LIBXL_TOOSTACK_DOMID; so there is no reason
to restrict this for network driver domains any more.

This is a candidate for backporting to 4.3.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Ian Jackson <ian.jackson@citrix.com>
CC: Jan Beulich <jbeulich@suse.com>