xl: make "xl list -l" proper JSON

xl: make "xl list -l" proper JSON

Bastian Blank reports that the output of this command is just multiple
JSON objects concatenated and is not a single properly formed JSON
object.

Fix this by wrapping in an array. This turned out to be a bit more
intrusive than I was expecting due to the requirement to keep
supporting the SXP output mode.

Python's json module is happy to parse the result...

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>