xen/sched: add scheduling granularity enum

xen/sched: add scheduling granularity enum

Add a scheduling granularity enum ("cpu", "core", "socket") for
specification of the scheduling granularity. Initially it is set to
"cpu", this can be modified by the new boot parameter (x86 only)
"sched-gran".

According to the selected granularity sched_granularity is set after
all cpus are online.

A test is added for all sched resources holding the same number of
cpus. Fall back to core- or cpu-scheduling in that case.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>