qemu: Allow creating ppc64 guests with graphics and no USB mouse

qemu: Allow creating ppc64 guests with graphics and no USB mouse

The existing behavior for ppc64 guests is to always add a USB
keyboard and mouse combo if graphics are present; unfortunately,
this means any attempt to use a USB tablet will cause both pointing
devices to show up in the guest, which in turn will result in poor
user experience.

We can't just stop adding the USB mouse or start adding a USB tablet
instead, because existing applications and users might rely on the
current behavior; however, we can avoid adding the USB mouse if a USB
tablet is already present, thus allowing users and applications to
create guests that contain a single pointing device.

https://bugzilla.redhat.com/show_bug.cgi?id=1683681

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>