CP-7835: merge blktap3

In blktap3 we re-engineer the data path and connect the front-end directly to tapdisk, so blkback and blktap are no longer necessary but still used to provide a way for dom0 tools to access the VM's data. As blkback can no longer be used to run the Xenbus protocol, a user-space daemon (tapback) is introduced with blktap3 to do that job.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>