virttest.staging.utils_cgroup: Remove '\t' from the string in get_property()

When call get_property() we may get following error n cgroup test:
Description of problem:
Traceback (most recent call last):
  File "/home/autotest/client/shared/test.py", line 426, in _exec
    _call_test_function(self.execute, *p_args, **p_dargs)
  File "/home/autotest/client/shared/test.py", line 846, in _call_test_function
    return func(*args, **dargs)
  File "/home/autotest/client/shared/test.py", line 299, in execute
    postprocess_profiled_run, args, dargs)
  File "/home/autotest/client/shared/test.py", line 216, in _call_run_once
    *args, **dargs)
  File "/home/autotest/client/shared/test.py", line 322, in run_once_profiling
    self.run_once(*args, **dargs)
  File "/home/autotest/client/tests/virt/virt.py", line 136, in run_once
    run_func(self, params, env)
  File "/home/autotest/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/home/autotest/client/tests/virt/qemu/tests/cgroup.py", line 2032, in run_cgroup
    return fce()
  File "/home/autotest/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/home/autotest/client/tests/virt/qemu/tests/cgroup.py", line 366, in blkio_throttle
    return blkio_throttle_multi([[_] for _ in speeds])
  File "/home/autotest/client/shared/error.py", line 138, in new_fn
    return fn(*args, **kwargs)
  File "/home/autotest/client/tests/virt/qemu/tests/cgroup.py", line 537, in blkio_throttle_multi
    % (dev[0], dev[1], speed))
  File "/home/autotest/client/tests/virt/virttest/staging/utils_cgroup.py", line 411, in set_property
    % (repr(check), repr(_values)))
TestError: cg.set_property(): Setting failed: desired = '8:16 1024', real values = ['8:16\t1024']
    [context: Executing test: blkio_throttle --> Init --> Setup test]

Signed-off-by: Feng Yang <fyang@redhat.com>