core: update CGroupBlockIODeviceBandwidth to record both rbps and wbps

CGroupBlockIODeviceBandwith is used to keep track of IO bandwidth limits for
legacy cgroup hierarchies.  Unlike the unified hierarchy counterpart
CGroupIODeviceLimit, a CGroupBlockIODeviceBandwiddth records either a read or
write limit and has a couple issues.

* There's no way to clear specific config entry.

* When configs are cleared for an IO direction of a unit, the kernel settings
  aren't cleared accordingly creating discrepancies.

This patch updates CGroupBlockIODeviceBandwidth so that it behaves similarly to
CGroupIODeviceLimit - each entry records both rbps and wbps limits and is
cleared if both are at default values after kernel settings are updated.