[PATCH 0 of 3] blktap3/libxl: add support for blktap3 in libxl

[Thanos Makatos <thanos.makatos@citrix.com>]

This patch series implements support for blktap3 in libxl.

Supporting blktap3 requires rather few changes:
1. We introduce a new disk back-end type (TAP3) and a new device kind (VBD3) to
   allow blktap3 to co-exist with blktap3. blktap2 remains the default back-end
   for tap devices. Switching in the future to blktap3 as the default handler
   of tap devices should be trivial.
2. libxl doesn't spawn the tapdisk process any more, as is the case for
   blktap2, since the tapback daemon is responsible for that. Thus, libxl only
   needs to write to XenStore the file/partition/whatever backing the virtual
   disk so that the tapback daemon can pass it to tapdisk.
3. Since there is no block device in dom0 any more, pygrub won't be able to
   boot from VHD files. To get around this problem, pygrub can use the NBD
   functionality (existing in blktap2.5): it can explicitly ask tapback to
   create a NDB in order to access the virtual disk. This functionality will be
   implemented in a future patch series. However, this will only work for Linux
   dom0's. As a generic solution that would work on any dom0, we could
   implement a simple protocol for data exchange between pygrub and tapdisk.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

---
Changed since v1:
  * Check whether tapback's control socket exists instead of checking whether
    a process named "tapback" exists.
  * Don't duplicate code for writing the type:/path/to/file to XenStore.