i386: allow to load initrd below 4 GB for recent linux

i386: allow to load initrd below 4 GB for recent linux

Since linux commit: cf8fa920cb42 ("i386: handle an initrd in highmem (version 2)")
linux has supported initrd up to 4 GB, but the header field
ramdisk_max is still set to 2 GB to avoid "possible bootloader bugs".

When use '-kernel vmlinux -initrd initrd.cgz' to launch a VM,
the firmware(it could be linuxboot_dma.bin) helps to read initrd
contents into guest memory(below ramdisk_max) and jump to kernel.
that's similar with what bootloader does, like grub.

In addition, initrd_max is uint32_t simply because QEMU doesn't support
the 64-bit boot protocol (specifically the ext_ramdisk_image field).

Therefore here just limit initrd_max to UINT32_MAX simply as well to
allow initrd to be loaded below 4 GB.

NOTE: it's possible that linux protocol within [0x208, 0x20c]
supports up to 4 GB initrd as well.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>