libxl: make it possible to explicitly specify default sched params

libxl: make it possible to explicitly specify default sched params

To do so we define a discriminating value which is never a valid real value for
each parameter.

While there:

 - removed libxl_sched_*_domain in favour of libxl_domain_sched_params.
 - use this new functionality for the various xl commands which set sched
   parameters, which saves an explicit read-modify-write in xl.
 - removed call of xc_domain_getinfolist from a few functions which weren't
   actually using the result (looks like a cut and paste error)
 - fix xl which was setting period for a variety of different config keys.

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>