CP-5050: Notify XAPI about critical coalesce errors

The rate XAPI is notified is throttled to avoid spamming (currently
defaults to one message per hour, per SR). This rate can be overridden
by setting the "coalesce-error-rate" key in an SR's other-config.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Suggested-by: Andrei Lifchits <andrei.lifchits@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #38 on xapi-project/sm