[PATCH 0 of 9 v2] libxl: add support for hotplug script calling from libxl

[Roger Pau Monne <roger.pau@entel.upc.edu>]

This patch series adds support for hotplug script calling directly
from libxl, instead of relying on xenbackendd. Also some patches
contain general bug fixes.

Currently Linux hotplug script call functions are empty, so Linux
continues to use udev rules to call hotplug scripts.

Patches 1, 7, 8, 9 are NetBSD specific, and basicaly pave the way for
the application of the bigger changes present in this series.

Patch 2 adds support for mounting raw image files using the vnd
device. Since NetBSD doesn't have qdisk or blktap support, the only
way to use raw images with guests is to mount the image and pass the
block device as a "PHY" backend. To check wheter an OS supports raw
images as "PHY" backends two new files are added to the project, to
avoid using #ifdefs, that contain a helper function. The file
to be included is decided during the compilation process.

Patch 3 adds a generic function to fork the current process and
execute a given file, which will be later used to execute hotplug
scripts synchronously.

Patches 4 and 5 add a new function to wait for a device to reach a
certain state, and replace wait_for_dev_destroy with this more generic
implementation. This function is also used to wait for device
initialization before calling hotplug scripts. The added function will
benefit from a rework after event support is added to libxl.

Finally patch 6 adds the calling of hotplug scripts when devices are
initializated and removed. Two new files are also added to support
hotplug scripts, since Linux and NetBSD hotplug scripts have different
call parameters. The path of the script to execute is retrieved
from xenstore. The file to include is also decided at compile
time.

Please review, Roger.