arch_init: align MR size to target page size

arch_init: align MR size to target page size

Migration code assumes that each MR is a multiple of TARGET_PAGE_SIZE:
MR size is divided by TARGET_PAGE_SIZE, so if it isn't migration
never completes.
But this isn't really required for regions set up with
memory_region_init_ram, since that calls qemu_ram_alloc
which aligns size up using TARGET_PAGE_ALIGN.

Align MR size up to full target page sizes, this way
migration completes even if we create a RAM MR
which is not a full target page size.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>