Utils_test: use utils.rmdirs() to replace shutil.rmtree().

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt-tests: fix a typo in libvirt_bench_domstate_switch_with_iozone test.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>