libxl: force netback to wait for hotplug execution before connecting

libxl: force netback to wait for hotplug execution before connecting

By writing an empty "hotplug-status" xenstore node in the backend path
libxl can force Linux netback to wait for hotplug script execution
before proceeding to the 'connected' state.

This is required so that netback doesn't skip state 2 (InitWait) and
thus blocks libxl waiting for such state in order to launch the
hotplug script (see libxl__wait_device_connection).

Reported-by: James Dingwall <james-xen@dingwall.me.uk>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: James Dingwall <james-xen@dingwall.me.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Tested-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>