CA-142415: Correct host level performance counters

When VM reboots, the performance counters such as iops-read /
io_throughput_read will reset to zero and recount. The VM level
counters could be handled properly, however the host level ones
which are aggregated from VM level ones are incorrect.

Solution: rrdp_iostats now will keep track the previous values
of the VDI level counters, and computes the right counters for
both VM/SR level counters based on them.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>