conf: make virNodeDevCapData an official type

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.