tools/hotplug: Add an initscript to start "xl devd" in a driver domain

tools/hotplug: Add an initscript to start "xl devd" in a driver domain

The removal of the udev rules highlighted that although it has been
replaced by "xl devd" there isn't an initscript to replace it.

To enable this add a --pidfile option to xl devd.

Tested on Linux by running the script in dom0 and checking the daemon
was started/stopped, but not in an actual driver domain environment
since I don't have one conveniently available. I also checked that
running without the --pidfile option still works.

Scripts mainly cribbed from the xencommons for each platform.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad@darnok.org>
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>