bql: Fix bql_locked status with condvar APIs

QEMU has a per-thread "bql_locked" variable stored in TLS section, showing
whether the current thread is holding the BQL lock.

It's a pretty handy variable.  Function-wise, QEMU have codes trying to
conditionally take bql, relying on the var reflecting the locking status
(e.g. BQL_LOCK_GUARD), or in a GDB debugging session, we could also look at
the variable (in reality, co_tls_bql_locked), to see which thread is
currently holding the bql.

When using that as a debugging facility, sometimes we can observe multiple
threads holding bql at the same time. It's because QEMU's condvar APIs
bypassed the bql_*() API, hence they do not update bql_locked even if they
have released the mutex while waiting.

It can cause confusion if one does "thread apply all p co_tls_bql_locked"
and see multiple threads reporting true.

Fix this by moving the bql status updates into the mutex debug hooks.  Now
the variable should always reflect the reality.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250904223158.1276992-1-peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

stubs/iothread-lock.c

@@ -34,3 +34,12 @@ void bql_block_unlock(bool increase)
     assert((new_value > bql_unlock_blocked) == increase);
     bql_unlock_blocked = new_value;
 }
+
+bool mutex_is_bql(QemuMutex *mutex)
+{
+    return false;
+}
+
+void bql_update_status(bool locked)
+{
+}