Add a b4 configuration file

Add a b4 configuration file

b4 [1] is a convenient tool to manage patch series with mailing list
working flow.

Add a project default config file to match QEMU's mailing list conventions
as well as adopting differences on scripting.

Examples of b4:

    ```
    $ b4 prep --check
    Checking patches using:
      scripts/checkpatch.pl -q --terse --no-summary --mailback -

    ---
    Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
    ---
    Changes in v2:
    - Add lore masks (philmd) from:
      https://lore.kernel.org/qemu-devel/20241224135054.10243-1-philmd@linaro.org/
    - Link to v1: https://lore.kernel.org/r/20241222-b4-config-v1-1-b3667beb30a4@flygoat.com
    ---
    ● cc5a4c890fed: Add a b4 configuration file
      ● checkpatch.pl: 27: WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
    ---
    Success: 0, Warning: 1, Error: 0
    ```

    ```
    $ b4 prep -c
    Will collect To: addresses using echo
    Will collect Cc: addresses using get_maintainer.pl
    Collecting To/Cc addresses
        + To: qemu-devel@nongnu.org
    ---
    You can trim/expand this list with: b4 prep --edit-cover
    Invoking git-filter-repo to update the cover letter.
    New history written in 0.02 seconds...
    Completely finished after 0.06 seconds
    ```

[1]: https://b4.docs.kernel.org/

Co-developed-by: Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-ID: <20250102-b4-config-v2-1-cc7299e399bb@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>