nodeinfo: use virDirRead API
    
    This makes sure that errno is reset before readdir is called, even if
    the loop does a 'continue'.
    
    This fixes issue with musl libc which sets errno on sscanf. The
    following 'continue' makes the errno be set before calling readdir.
    
    Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
    Signed-off-by: Eric Blake <eblake@redhat.com>