libxc: add rtds scheduler

libxc: add rtds scheduler

Add xc_sched_rtds_* functions to interact with Xen to set/get domain's
parameters for rtds scheduler.
Note: VCPU's information (period, budget) is in microsecond (us).

Signed-off-by: Meng Xu <mengxu@cis.upenn.edu>
Signed-off-by: Sisu Xi <xisisu@gmail.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
[ ijc -- xenctrl.h has moved to tools/libxc/include, adjust patch to match ]