Rename "QEMU global mutex" to "BQL" in comments and docs

The term "QEMU global mutex" is identical to the more widely used Big
QEMU Lock ("BQL"). Update the code comments and documentation to use
"BQL" instead of "QEMU global mutex".

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Message-id: 20240102153529.486531-6-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/devel/qapi-code-gen.rst

@@ -594,7 +594,7 @@ blocking the guest and other background operations.
 Coroutine safety can be hard to prove, similar to thread safety.  Common
 pitfalls are:
 
-- The global mutex isn't held across ``qemu_coroutine_yield()``, so
+- The BQL isn't held across ``qemu_coroutine_yield()``, so
   operations that used to assume that they execute atomically may have
   to be more careful to protect against changes in the global state.