virtio-net: don't allocate control_buf if not supported

Not all virtio_net devices support the ctrl queue feature. Thus, there
is no need to allocate unused resources.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20210502093319.61313-1-mgurtovoy@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>