(Refactored) VHD Support Addition

[PATCH 0/4] (Refactored) VHD Support addition
The patches that follow implement Microsoft's VHD format support, and apply to xen-unstable.hg CS 15414:3827b12d24dd
Per Mark McLoughlin's suggestions - these have been reworked to use the tapaio wrapper.

This is based on the VHD format spec covered by Microsoft's "Open Specification Promise":
http://www.microsoft.com/interop/osp/default.mspx
MS VHD Spec can be found at the following link:
http://download.microsoft.com/download/f/f/e/ffef50a5-07dd-4cf8-aaa3-442c0673a029/Virtual%20Hard%20Disk%20Format%20Spec_10_18_06.doc

It is divided up into the following patches:

qemu-vhd-support.patch        [1/4]
tapdisk-vhd-support.patch     [2/4]
blktap-fixes.patch            [3/4]
vdisk-support.patch           [4/4]

### libvdisk ####
These patches implement a shared library (libvdisk) between qemu, and blktap which can be expanded upon for additional formats in the future. This was done in order to avoid code duplcation between the two areas. At this time, it only supports the vhd format.

### vdisk_tool ###
The vdisk_tool provides a means of creating, and converting vhd files. Like libvdisk, it is designed with future support of additional formats in mind, but currently only supports vhd.

Usage: vdisk_tool OPTIONS -# <format-specific options> <filename>
OPTIONS:
[-f <format>] [-C] [-H] [-M] [-D <block> [-b <num_blocks>] -o outfile]]
-C Create a vdisk
-H Read vdisk headers from file
-M Modify a vdisk
-D Dump a vhd
block first block to read (required)
num_blocks number of blocks to read. If not
specified, whole file will be read
outfile output file. If not specified,
stdout is used

With this, you can convert foo.vhd to raw volume /dev/foo/bar:
vdisk_tool -D 0 -o /dev/foo/bar foo.vhd

To convert foo.vhd to fixed bar.vhd
vdisk_tool -D 0 -o bar.vhd foo.vhd
vdisk_tool -C -# -S <sizeB> -f bar.vhd

To read blocks 2 through 6 from foo.vhd to stdout:
vdisk_tool -D 2 -b 5 foo.vhd