Add -usbdevice tablet to KVM when using vnc

When using VNC, it is recommended to use a tablet-style
input device instead of a mouse. This allows most VNC viewers
to send proper mouse coordinates to the virtual machine's desktop
resulting in perfectly aligned guest and host mouse pointers.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>