Remove dummy plugin

There's a more up-to-date version in the rrdd-plugin library.

Signed-off-by: John Else <john.else@citrix.com>
Remove unneeded module open

Signed-off-by: John Else <john.else@citrix.com>
Remove Rrdp_common and switch to Rrdd_plugin

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #15 from johnelse/use-ocaml-rrdd-plugin

Remove Rrdp_common and Rrdp_dummy, switch to Rrdd_plugin
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>
Merge pull request #16 from kaifengz/CA-142415

CA-142415: Correct host level performance counters
CA-142270: Include blktap3 perf data to host level stats

Host level metrics were accumulated from the VDI level stats.  Prior XS
6.5, the VDI level stats were read from sysfs. With blktap3, stats
exposed under /dev/shm should also be taken into account.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>