[PATCH 0/5]: Expand xvd to support > 16 devices v2

[Chris Lalancette <clalance@redhat.com>]

Current blktap and blkfront are limited to 16 xvd devices (xvda -> xvdp).
This is enforced in the userland dom0 tools, but is also hard-coded into the
blkfront kernel code (even though modern dev_t can hold many more than 256
minors).  Based on the discussion that we had here:

http://lists.xensource.com/archives/html/xen-devel/2008-05/msg00128.html

I ended up implementing Ian Jackson's suggestion here:

http://lists.xensource.com/archives/html/xen-devel/2008-05/msg00231.html

Basically, I left the old format alone, but added a new format that looks like:

 1 << 28 | disk << 8 | partition       xvd, disks or partitions 16 onwards

This format is used for any disks xvdq onward.  Note that blktap has a hardcoded
limit of 100 devices that I did not change with this patch series; if that ends
up being a problem, then that's just a simple #define to change.

I did not expand the number of partitions available (it's still 15), although
there is space in the allocation to do that if someone wishes.  More details are
in each individual patch.

Note that I developed this against RHEL-5 kernels and ported it over to
xen-unstable, and only compile tested it there.

Changes since v1:
- Fix up the python to use a more pythonic piece of code in main.py (thanks
Brendan Cully)
- Re-structure the blkfront code to be much simpler
- Add some additional error checking in the xlvbd_add() path (details in the patch)
- Add a Changelog entry

Chris Lalancette