cgroup.blkio_bandwidth test should set up VM disk with cache=none

[rayx]

The test creates two VMs and sets up cgroup blkio.weight parameter for each of their qemu processes. The ratio of weight used in test is 100:1000. However, the result is not as expected. The actual ratio of I/O speed in the two VMs is about 1:1.

status norm_weights    norm_out       actual

FAIL   0.0909090909091 0.497569739009 897372

FAIL   0.909090909091  0.502430260991 906138

I believe one factor that leads to the false failure is that VM disk image file should be set up with "cache=none" option.