spapr: Enable use of huge pages

spapr: Enable use of huge pages

0b183fc87 "memory: move mem_path handling to
memory_region_allocate_system_memory" disabled -mempath use for all
machines that do not use memory_region_allocate_system_memory() to
register RAM. Since SPAPR uses memory_region_init_ram(), the huge pages
support was disabled for it.

This replaces memory_region_init_ram()+vmstate_register_ram_global() with
memory_region_allocate_system_memory() to get huge pages back.

This changes RAM size from (ram_limit - rma_alloc_size) to ram_limit as
the previous patch moved RMA memory region allocation after RAM allocation
and therefore this change does not have immediate effect but simplifies
the code.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>