qemu-system-ppc64 by default has non-working keyboard

Compile qemu from git and do:

  ./ppc64-softmmu/qemu-system-ppc64

(ie. no parameters).  It boots to an OpenBIOS prompt.  However the keyboard doesn't work.  After ~10 keypresses, qemu just says:

usb-kbd: warning: key event queue full
usb-kbd: warning: key event queue full
usb-kbd: warning: key event queue full
usb-kbd: warning: key event queue full

There is no indication inside the guest that OpenBIOS is seeing keyboard events.

Also there's no indication of what type of keyboard devices are available, nor what we should use.