feature #1678: monitor system ds probe for kvm and xen

I'm not happy passing parameters as argument possition as these
are also used for collectd. As soon as we add a new parameter
the probe should be changed to get another ARG possition.

A better way could be adding parameters in the form:

  DATASTORE_LOCATION=/my/datastores OTHER_VAR=other_value

Parsing this and set the environment with those values.

No time, no shine...