libxl: call hotplug scripts for nic devices from libxl

libxl: call hotplug scripts for nic devices from libxl

Since most of the needed work is already done in previous patches,
this patch only contains the necessary code to call hotplug scripts
for nic devices, that should be called when the device is added or
removed from a guest.

Added another parameter to libxl__get_hotplug_script_info, that is
used to know the number of times hotplug scripts have been called for
that device. This is currently used by IOEMU nics on Linux.

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