xen/cpupool: support moving domain between cpupools with different granularity

When moving a domain between cpupools with different scheduling
granularity the sched_units of the domain need to be adjusted.

Do that by allocating new sched_units and throwing away the old ones
in sched_move_domain().

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