virtio: use fast-path for ethernet-input if possible

If a frame should be sent to 'ethernet-input' set the
ETH_INPUT_FRAME_F_SINGLE_SW_IF_IDX flag. It will force 'ethernet-input'
to use a fast-path for such frames.
This patch also aligns the behaviour with other input nodes.

Type: improvement

Change-Id: Icff0fa31204d5304a2ea0a4f4e7bc418dedbfe32
Signed-off-by: Stanislav Zaikin <zstaseg@gmail.com>