libxl: make hotplug execution conditional on backend_domid == domid

libxl: make hotplug execution conditional on backend_domid == domid

libxl currently refuses to execute hotplug scripts if the backend
domid of a device is different than LIBXL_TOOLSTACK_DOMID. This will
prevent libxl from executing hotplug scripts when running on a domain
different than LIBXL_TOOLSTACK_DOMID, we should instead check if
backend_domid is different than current domid.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>