hw/loongarch/boot: Adjust the loading position of the initrd

hw/loongarch/boot: Adjust the loading position of the initrd

When only the -kernel parameter is used to load the elf kernel, the initrd
is loaded in the ram. If the initrd size is too large, the loading fails,
resulting in a VM startup failure. This patch first loads initrd near
the kernel.

When the nearby memory space of the kernel is insufficient, it tries to
load it to the starting position of high memory. If there is still not
enough, qemu will report an error and ask the user to increase the memory
space for the virtual machine to boot.

Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Message-Id: <20250506080946.817092-1-lixianglai@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>