conf: add virDomainDefAddController()

conf: add virDomainDefAddController()

We need a virDomainDefAddController() that doesn't check for an
existing controller at the same index (since USB2 controllers must be
added in sets of 4 that are all at the same index), so rather than
duplicating the code in virDomainDefMaybeAddController(), split it
into two functions, in the process eliminating existing duplicated
code that loops through the controller list by calling
virDomainControllerFind(), which does the same thing).