PR-1455: Update perfmon wrt the new alert priority grade

UPDATE: Unfortunately the changeset was lost when being ported to the
clearwater branch, now we spot this error and put it into Creedence.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Update the default priority setting in the comment section

... to reflect the changes of PR-1455.

Signed-off-by: Zheng Li <dev@zheng.li>
CP-9091: Add Dom0 mem_usage alert for perfmon

Signed-off-by: Zheng Li <dev@zheng.li>
CP-9093: decrease the inhibit period of fs_usage alert

It seems that the inhibit period of fs_usage alert is set too long (one week).
The consequence is that the alerts will be sent out much less frequently
(not showing much emergency as we want). Also it will mute the alarm for a week
long even if a different occurence shows up later on (after the first occurence
being addressed already) during this period of time.

Signed-off-by: Zheng Li <dev@zheng.li>
CP-9091: Add mem_usage logic in mail-alarm

Signed-off-by: Zheng Li <dev@zheng.li>
CP-9093: fix a debug message error

Signed-off-by: Zheng Li <dev@zheng.li>
Merge pull request #1846 from zli/xenserver/xs64bit-ring3/CP-9091

[xs64bit] Add Dom0 mem_usage alert