-device nec-usb-xhci (usb 3) breaks VM snapshots

Enabling the USB 3.0 controller apparently breaks "live" snapshotting.

To reproduce:

    $ qemu-system-x86_64 -device nec-usb-xhci vm.qcow2

then, at the Monitor:

    (qemu) savevm
    Error -22 while writing VM
    (qemu) 

Instead, if I remove -device nec-usb-xhci from the cmdline, everything works fine.

Some QEMU and OS info:

    $ qemu-system-x86_64 -version
    QEMU emulator version 1.5.0, Copyright (c) 2003-2008 Fabrice Bellard

    $ uname -a
    Linux localhost 3.2.0-4-amd64 #1 SMP Debian 3.2.41-2 x86_64 GNU/Linux

The same also happens with 1.4.2. All compiled from source, not Debiabn package (btw, this *also* happens with distro packages/older versions).