CP-7396: Ensure tapdisk reports metrics to rrdd.

- Register tapdisk with rrdd as a plugin when we open a new tapdisk.
- Deregister tapdisk with rrdd before we detach the tapdisk.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #75 on xapi-project/sm