: Greater than 16 xvd fixes

All,
As pointed out by Christopher Head, the ioctl I hijacked with the extended
block device numbers works on 64-bit, but actually fails on 32-bit. Additionally, Keir pointed out that we should really maintain backwards compatibility so that older dom0 kernels will work with newer tools and vice-versa. Finally, I found a bug in the blkfront implementation of the extended devices. All 3 of these issues should be addressed in the following patches. Note again that I developed these on RHEL-5, and only compile-tested against xen-unstable. Further note that I didn't check if these work on 32-bit, so if someone could give them a whirl there, it would be appreciated.