Reinstate io_throughput_* datasources for backwards compat.
Reinstate io_throughput_* datasources for backwards compat.

This is a partial revert of:

    commit 646e54b7d0798f63e6612af355acd0cf47063739
    "CA-184525: Change set of datasources exported by xcp-rrdd-iostat"

This commit reverts just the negative part of the diff for 646e54b.

Signed-off-by: Si Beaumont <a0992b7f1f450b4c7716d34b81a2d6f0877ae11a@citrix.com>

johnelse/rrdd-plugins,johnelse/rrdd-plugins