[PATCH 0 of 9 RFC v2] blktap3: Introduce a small subset of blktap3 files

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

blktap3 is a disk back-end driver. It is based on blktap2 but does not require
the blktap/blkback kernel modules as it allows tapdisk to talk directly to
blkfront. This primarily simplifies maintenance, and _may_ lead to performance
improvements. blktap3 is based on a blktap2 fork maintained mostly by Citrix
(it lives in github), so these changes are also imported, apart from the
blktap3 ones.

I've organised my upstream effort as follows:
1. Upstream the smallest possible subset of blktap3 that will allow guest VMs
   to use RAW images backed by blktap3. This will enable early testing on the
   bits introduced by blktap3.
2. Upstream the remaining of blktap3, most notably back-end drivers, e.g. VHD
   etc.
3. Import bug fixes from blktap2 living in github.
4. Import new features and optimisations from blktap2 living in github, e.g.
   the mirroring plug-in.

blktap3 is made of the following components:
1. blkfront (not a blktap3 component and already upstream): a virtual
   block device driver in the guest VM that receives block I/O requests and
   forwards them to tapdisk via the shared ring.
2. tapdisk: a user space process that receives block I/O requests from
   blkfront, translates them to whatever the current backing file format is
   (i.e. RAW, VHD, qcow etc.), and performs the actual I/O. Apart from block
   I/O requests, the tapdisk also allows basic management of each virtual
   block device, e.g. a device may be temporarily paused. tapdisk listens to
   a loopback socket for such commands. The tap-ctl utility (explained later)
   can be used for managing the tapdisk.
3. libtapback: a user space library that implements the functionality required
   to access the shared ring. It is used by tapdisk to obtain the block I/O
   requests forwarded by blkfront, and to produce the corresponding responses.
   This is the very "heart" of blktap3, it's architecture will be thoroughly
   explained by the patch series that introduces it.
4. tapback: a user space daemon that acts as the back-end of each virtual
   block device: it monitors XenStore for the block front-end's state changes,
   creates/destroys the shared ring, and instructs the tapdisk to connect
   to/disconnect from the shared ring. It also communicates to the block
   front-end required parameters (e.g. block device size in sectors) via
   XenStore.
5. libblktapctl: a user space library where the tapdisk management functions
   are implemented.
6. tap-ctl: a user space utility that allows management of the tapdisk, uses
   libblktapctl.

The tapdisk is spawned/destroyed by libxl when a domain is created/destroyed,
in the exact same way as in blktap2. libxl uses libblktapctl for this.

This patch series introduces a small subset of files required by tapback (the
tapback daemon is introduced by the next patch series):
- basic blktap3 header files
- a rudimentary implementation of libblktapctl. Only the bits required by
  tapback to manage the tapdisk are introduced, the rest of this library will
  be introduced by later patches.

---
Changed since v1:
* In all patches the patch message has been improved.
* Patches 1, 5, and 6 use GPLv2.
* Patch 0: Basic explanation of blktap3's fundamental components.
* Patch 9: Improved tools/blktap3/control/Makefile by moving hard coded
  paths to config/StdGNU.mk.