[PATCH 0 of 4 v2] 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. Booting with pygrub from VHD files is trivially done by attaching blkfront
   to dom0.

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.

Changed since v2:
  * Support for booting from RAW and VHD images using pygrub.