bql: add a "mock" BQL for Rust unit tests

Right now, the stub BQL in stubs/iothread-lock.c always reports itself as
unlocked.  However, Rust would like to run its tests in an environment where
the BQL *is* locked.  Provide an extremely dirty function that flips the
return value of bql_is_locked() to true.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/meson.build

@@ -60,7 +60,7 @@ test('rust-qemu-api-integration',
         dependencies: [qemu_api, qemu_api_macros],
         link_whole: [rust_qemu_api_objs, libqemuutil]),
     args: [
-        '--test',
+        '--test', '--test-threads', '1',
         '--format', 'pretty',
     ],
     protocol: 'rust',