core: introduce CGroupIOLimitType enums

Currently, there are two cgroup IO limits, bandwidth max for read and write,
and they are hard-coded in various places.  This is fine for two limits but IO
is expected to grow more limits - low, high and max limits for bandwidth and
IOPS - and hard-coding each limit won't make sense.

This patch replaces hard-coded limits with an array indexed by
CGroupIOLimitType and accompanying string and default value tables so that new
limits can be added trivially.