libqos: add qvirtqueue_cleanup()

qvirtqueue_setup() allocates the vring and virtqueue state.  So far
there has been no function to free it.  Callers have been using
guest_free() for the vring but forgot to free the QVirtQueue state.

This patch solves the memory leak by introducing qvirtqueue_cleanup().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
