vdpa/snet: use likely/unlikely macros in hot functions

- kick callback: most likely that the VQ is ready.
- interrupt handlers: most likely that the callback is not NULL.

Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
Message-Id: <20230409120242.3460074-1-alvaro.karsz@solid-run.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>