sched_rt: print useful affinity info when dumping

sched_rt: print useful affinity info when dumping

In fact, printing the cpupool's CPU online mask
for each vCPU is just redundant, as that is the
same for all the vCPUs of all the domains in the
same cpupool, while hard affinity is already part
of the output of dumping domains info.

Instead, print the intersection between hard
affinity and online CPUs, which is --in case of this
scheduler-- the effective affinity always used for
the vCPUs.

This change also takes the chance to add a scratch
cpumask area, to avoid having to either put one
(more) cpumask_t on the stack, or dynamically
allocate it within the dumping routine. (The former
being bad because hypervisor stack size is limited,
the latter because dynamic allocations can fail, if
the hypervisor was built for a large enough number
of CPUs.) We allocate such scratch area, for all pCPUs,
when the first instance of the RTDS scheduler is
activated and, in order not to loose track/leak it
if other instances are activated in new cpupools,
and when the last instance is deactivated, we (sort
of) refcount it.

Such scratch area can be used to kill most of the
cpumasks{_var}_t local variables in other functions
in the file, but that is *NOT* done in this chage.

Finally, convert the file to use keyhandler scratch,
instead of open coded string buffers.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>