Blkio isolator: handled missing CFQ statistics.

[cf-natali]

CFQ was removed from 5.x kernel, so we need to silently ignore missing CFQ statistics.

Before:

[----------] 1 test from CgroupsIsolatorTest [ RUN      ]
[...]
CgroupsIsolatorTest.ROOT_CGROUPS_BlkioUsage W0824 21:58:36.604790 308667
cgroups.cpp:932] Skipping resource statistic for container
b2d67073-85fa-4a0b-84a3-790791047eac because: Failed to read from
'blkio.time': No such file or directory
../../src/tests/containerizer/cgroups_isolator_tests.cpp:2656: Failure
Value of: usage->has_blkio_statistics() Actual: false Expected: true [
[...]
FAILED  ] CgroupsIsolatorTest.ROOT_CGROUPS_BlkioUsage (1888 ms)