sched: get rid of the per domain vCPU list in RTDS

sched: get rid of the per domain vCPU list in RTDS

As, curently, there is no reason for bothering having
it and keeping it updated.

In fact, it is only used for dumping and changing
vCPUs parameters, but that can be achieved easily with
for_each_vcpu.

While there, take care of the case when
XEN_DOMCTL_SCHEDOP_getinfo is called but no vCPUs have
been allocated yet (by returning the default scheduling
parameters).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>