net/virtio-user: fix missing default config value

net/virtio-user: fix missing default config value

With current config structure, all configuration parameters put into
common_base with a default value, and overwritten in environment file
if required, CONFIG_RTE_VIRTIO_USER is missing in common_base.

This fix is simple, by adding CONFIG_RTE_VIRTIO_USER=n as the default
macro value.

Fixes: ce2eabdd43ec ("net/virtio-user: add virtual device")

Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>