libxl: expose a single get/setter for domain scheduler parameters

libxl: expose a single get/setter for domain scheduler parameters

This is consistent with having a single struct for all parameters.

Effectively renames and exports libxl__sched_set_params as
libxl_domain_sched_params_set. libxl_domain_sched_params_get is new.

Improve const correctness of the setters while I'm here.

Use shorter LOG macros when touching a line anyway.

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>