libxl: add device backend listener in order to launch backends

libxl: add device backend listener in order to launch backends

Add the necessary logic in libxl to allow it to act as a listener for
launching backends in a driver domain, replacing udev (like we already
do on Dom0). This new functionality is acomplished by watching the
domain backend path (/local/domain/<domid>/backend) and reacting to
device creation/destruction.

The way to launch this listener daemon is from xl, using the newly
introduced "devd" command. The command will daemonize by default,
using "xldevd.log" as it's logfile. Optionally the user can force the
execution of the listener in the foreground by passing the "-F"
option to the devd command.

Current backends handled by this daemon include Qdisk, vbd and vif
device types.

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