net/virtio-user: fix return value not checked

net/virtio-user: fix return value not checked

When return values of function calls are not checked, Coverity will
report errors like:

    if (rte_kvargs_count(kvlist, VIRTIO_USER_ARG_PATH) == 1)
    >>>     CID 127477:    (CHECKED_RETURN)
    >>>     Calling "rte_kvargs_process" without checking return value
            (as is done elsewhere 25 out of 30 times).
         		rte_kvargs_process(kvlist, VIRTIO_USER_ARG_PATH,
         				   &get_string_arg, &path);

Coverity issue: 127477, 127478
Fixes: ce2eabdd43ec ("net/virtio-user: add virtual device")
Fixes: 6a84c37e3975 ("net/virtio-user: add vhost-user adapter layer")
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>