In perfmon: add a workaround for CA-97947, and fix error-logging

In CA-97947 the xapi rpc call that creates the alert message has
become case-sensitive.
The workaround is to use the capitalisation it recognises.
Also the error-logging in perfmon could not handle Exceptions of
type XenAPI.Failure; now it can and is more robust.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>