ppc: fix -mem-path failure

commit e938ba0c tried to enable -mem-path for ppc but breaked some ppc
boards.

The problems are:

1. it fails when allocating memory for rom, sram whose sizes are less
   than huge page size:

   ./ppc-softmmu/qemu-system-ppc  -m 512 -mem-path /hugepages/ \
   -kernel /home/hutao/Downloads/vmlinux-ppc -initrd \
   /home/hutao/Downloads/initrd-ppc.gz
   qemu-system-ppc: /mnt/data/projects/qemu/exec.c:1184: qemu_ram_set_idstr: Assertion `new_block' failed.

2. if there is a numa node backed by memory backend object, qemu fails
   with message:

   ./ppc-softmmu/qemu-system-ppc  -m 512 \
   -object memory-backend-file,size=512M,mem-path=/hugepages,id=f0 \
   -numa node,nodeid=0,memdev=f0 \
   -kernel /home/hutao/Downloads/vmlinux-ppc \
   -initrd /home/hutao/Downloads/initrd-ppc.gz
   qemu-system-ppc: memory backend f0 is used multiple times. Each -numa option must use a different memdev value.

This patch does following:

1. replaces memory_region_allocate_system_memory() with
   memory_region_init_ram() for rom, sram. Then only system memory
   is backed by hugepages when specifying mem-path.

2. for memory banks, allocates all ram with
   one memory_region_allocate_system_memory(), and use
   memory_region_init_alias() to initialize memory banks.

Tested machines: default(g3beige), mac99, taihu, bamboo, ref405ep.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
