Allow configuring thresholds for XS alarms

[julien-f]

XS emits alarms, special messages when values goes above a configured threshold.

Example:

Name	Value
mem_usage	0.950917
alarm_trigger_level	0.95
alarm_trigger_period	60
alarm_auto_inhibit_period	3600

[olivierlambert]

And how do we modify those thresholds?

[julien-f]

No idea, we'll have to look how XenCenter does it.