large -initrd can wrap around in memory causing memory corruption

We don't use large -initrd in libguestfs any more, but I noticed that a large -initrd file now crashes qemu spectacularly:

$ ls -lh /tmp/kernel /tmp/initrd 
-rw-r--r--. 1 rjones rjones 273M Jun  3 14:02 /tmp/initrd
lrwxrwxrwx. 1 rjones rjones   35 Jun  3 14:02 /tmp/kernel -> /boot/vmlinuz-3.9.4-200.fc18.x86_64

$ ./x86_64-softmmu/qemu-system-x86_64 -L pc-bios \
    -kernel /tmp/kernel -initrd /tmp/initrd -hda /tmp/test1.img -serial stdio \
    -append console=ttyS0

qemu crashes with one of several errors:

PFLASH: Possible BUG - Write block confirm

qemu: fatal: Trying to execute code outside RAM or ROM at 0x00000000000b96cd

If -enable-kvm is used:

KVM: injection failed, MSI lost (Operation not permitted)

In all cases the SDL display fills up with coloured blocks before the crash (see the attached screenshot).