Data races in test-bdrv-drain test
Description of problem:
Data races in the access of `Job` fields in the `test-bdrv-drain` test 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_=186 G_TEST_SRCDIR=$QEMU_BUILD_DIR/tests/unit G_TEST_BUILDDIR=$QEMU_BUILD_DIR/tests/unit $QEMU_BUILD_DIR/tests/unit/test-bdrv-drain --tap -k
```