net/virtio-user: adding link speed parameter

net/virtio-user: adding link speed parameter

virtio driver already parses speed devarg. virtio-user should add
it to list of valid devargs and call eth_virtio_dev_init function
which init speed value.

eth_virtio_dev_init already is called from virtio_user_pmd_probe
function. The only change is required to enable speed devargs:
adding speed to list of valid devargs.

Signed-off-by: Ivan Dyukov <i.dyukov@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>