xen/domain: Reduce the quantity of initialisation for system domains

xen/domain: Reduce the quantity of initialisation for system domains

 * System domains don't need watchdog initialisation or iomem/irq rangesets,
   and will not plausibly be a xenstore or hardware domain.
 * The idle domain doesn't need scheduler initialisation (and in particular,
   removing this path allows for substantial scheduler cleanup), and isn't
   liable to ever need late_hwdom_init().

Move all of these initialisations pass the DOMCRF_dummy early exit, and into
non-idle paths.  rangeset_domain_initialise() remains because it makes no
allocations, but does initialise a linked list and spinlock.  The poolid
parameter can be dropped as sched_init_domain()'s parameter is now
unconditionally 0.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>