Switch blktap licensing to GPLv2

[AnthoineB]

The Vates storage team would like to add support of VDI above 2TB.
To allow this we'd like to support Qcow2 format in the blktap component.
To help the development of this feature, we would like to import Qcow2 code from the Qemu implementation.

Since Qemu's code is licensed under GPLv2, we would like to also switch the global blktap license to GPLv2 to avoid license incompatibility.

Therefore, I am contacting all external contributors to get their approval on their commits to make this switch.
Only one contributor didn't answer (see update below).
In the meantime, this PR is open to review.

There are 2 parts in the PR:

1. Rewrite of BSD code coming from other components that we can convert (atomicio.c, list.h, aio_genevents.c, autogen.sh)
2. Conversion of our code base in GPLv2 by partially revert 3480fc8

Update:
I add 2 patches on the PR:

1. Remove code from Anthony Messerli as he didn't give his approval
2. Switch license text to SPDX as suggested in the comments.

[nraynaud]

I approve.

[TSnake41]

Could we use SPDX license identifiers instead (https://spdx.dev/learn/handling-license-info/) ?

[MarkSymsCtx]

Most of this appears to file header changes, which are not strictly necessary as the BSD license is compatible with the GPL and projects containing GPL code can safely contain files which are BSD licensed (the reverse of course not being true). Only the files which absolutely have to be GPL, i.e. the qcow2 management pieces that you will be adding, need to have GPL headers. The overall project license file should be updated to state that it is GPL2 with some BSD content.

[AnthoineB]

Most of this appears to file header changes, which are not strictly necessary as the BSD license is compatible with the GPL and projects containing GPL code can safely contain files which are BSD licensed (the reverse of course not being true). Only the files which absolutely have to be GPL, i.e. the qcow2 management pieces that you will be adding, need to have GPL headers. The overall project license file should be updated to state that it is GPL2 with some BSD content.

Version 3 of GPLv2 blanch pushed.

• I removed 4 commits that were reverting files to previous version holding GPLv2 license.
• I modified 2 commits:
  1. I don't change the headers of every files anymore and I add a comment in README.md about the dual-license.
  2. I change the SPDX to BSD-3-Clause.
• And I added 3 commits:
  1. The first one adds spdk headers in the list of other license files in LICENSE.
  2. The second one adds the copyright and license header to the xattr.c file.
  3. The third one updates the copyright, date and license of XenSource Inc. to Citrix Systems, Inc. to align with other files.

What do you think @MarkSymsCtx ?

[guzu]

Beside the conflict with atomicio, which will ultimately be removed by this PR, it looks ok to me.