virtio-gpu: move iov free to virtio_gpu_cleanup_mapping_iov

For symmetry reasons: virtio_gpu_create_mapping_iov() allocates it so
virtio_gpu_cleanup_mapping_iov() should free it, otherwise it's easy to
miss a free() needed and leak memory.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
