conf: make virNodeDevCapData an official type
    
    For some reason a union (_virNodeDevCapData) that had only been
    declared inside the toplevel struct virNodeDevCapsDef was being used
    as an argument to functions all over the place. Since it was only a
    union, the "type" attribute wasn't necessarily sent with it. While
    this works, it just seems wrong.
    
    This patch creates a toplevel typedef for virNodeDevCapData and
    virNodeDevCapDataPtr, making it a struct that has the type attribute
    as a member, along with an anonymous union of everything that used to
    be in union _virNodeDevCapData. This way we only have to change the
    following:
    
      s/union _virNodeDevCapData */virNodeDevCapDataPtr /
    
    and
    
      s/caps->type/caps->data.type/
    
    This will make me feel less guilty when adding functions that need a
    pointer to one of these.