virtio-net: Do not specify VIRTIO_RING_F_INDIRECT_DESC twice.

VIRTIO_RING_F_INDIRECT_DESC belongs to the base features bits. No need
to specify it in net driver.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Reviewed-by: Dor Laor <dor@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>