[PATCH 00 of 11 v3] 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, 10 are NetBSD specific, and basicaly pave the way 
for the application of the bigger changes present in this series.

Patch 11 is a trivial update for an error message.

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.

Changes since v2:

 * Use libxl_linux.c and libxl_netbsd.c to place OS specific 
   functions.

 * Fixed libxl__forkexec comment, adding the need to pass NULL as the 
   last array member.

 * Use XenbusState instead of state numbers.

 * Use libxl_report_child_exit and return the exit status of the 
   process in libxl__forkexec.

 * Pass action to execute to hotplug scripts, either CONNECT or 
   DISCONNECT.

Please review, Roger.