When CONFIG_RTE_VIRTIO_USER is disabled (default on FreeBSD),
the virtio driver cannot be compiled:
librte_pmd_virtio.a(virtio_ethdev.o): In function `eth_virtio_dev_init':
(.text+0x1eba): undefined reference to `virtio_user_ops'
Reported-by: Andrew Rybchenko <arybchenko@solarflare.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>