turn out usb_tablet for libvirt

This patch turns out usb_tablet for the libvirt nova.conf file.
When usb_tablet is turned on, qemu will poll for usb events and
this causes CPU usage even when the qemu is idle.

Change-Id: I03e260dfd0873b7d15e01c10c206203833d04e73
Merge "turn out usb_tablet for libvirt"