xl: error if vif backend!=0 is used with run_hotplug_scripts

xl: error if vif backend!=0 is used with run_hotplug_scripts

Print an error and exit if backend!=0 is used in conjunction with
run_hotplug_scripts. Currently libxl can only execute hotplug scripts
from the toolstack domain (the same domain xl is running from).

Added a description and workaround of this issue on
xl-network-configuration.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>