libxl: add internal function to get a domain's scheduler

libxl: add internal function to get a domain's scheduler

This takes into account cpupools.

Add a helper to get the info for a single cpu pool, refactor libxl_list_cpupool
t use this. While there fix the leaks due to not disposing the partial list on
realloc failure in that function.

Fix the failure of sched_domain_output to free the poolinfo list.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>