Data Race in data->cb() call and cb assignment in test-aio-multithread
Description of problem:
Potential data races between the `data->cb()` call and the assignment of `cb` in `test-aio-multithread` were identified using TSAN.
Steps to reproduce:
```sh
QEMU_BUILD_DIR=<path to the QEMU build directory>
QEMU_DIR=<path to the QEMU repository directory>
configure --enable-tsan --cc=clang --cxx=clang++ --enable-trace-backends=ust --enable-fdt=system --disable-slirp
make tests/unit/test-bdrv-drain
MALLOC_PERTURB_=102 G_TEST_SRCDIR=$QEMU_BUILD_DIR/tests/unit G_TEST_BUILDDIR=$QEMU_BUILD_DIR/tests/unit $QEMU_BUILD_DIR/tests/unit/test-aio-multithread --tap -k
```