Patch in blktap kernel driver from xenserver

This allows us to use blktap2.5 and therefore get decent
copy-on-write disks via VHDs (see http://github.com/xapi-project/blktap)

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #25 from jonludlam/blktap

Patch in blktap kernel driver from xenserver