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>
---
● 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/

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
---
 .b4-config | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/.b4-config b/.b4-config
new file mode 100644
index 0000000000000000000000000000000000000000..5feac15642958350c457d1769bf881184ae1a79f
--- /dev/null
+++ b/.b4-config
@@ -0,0 +1,10 @@
+#
+# Common b4 settings that can be used to send patches to QEMU upstream.
+# https://b4.docs.kernel.org/
+#
+
+[b4]
+    send-series-to = qemu-devel@nongnu.org
+    send-auto-to-cmd = echo
+    send-auto-cc-cmd = scripts/get_maintainer.pl --noroles --norolestats --nogit --nogit-fallback
+    prep-perpatch-check-cmd = scripts/checkpatch.pl -q --terse --no-summary --mailback -

---
base-commit: 65cb7129f4160c7e07a0da107f888ec73ae96776
change-id: 20241222-b4-config-e469b075802c

Best regards,
-- 
Jiaxun Yang <jiaxun.yang@flygoat.com>