This replaces the virPCIKnownStubs string array that was used
internally for stub driver validation.

Advantages:

  * possible values are well-defined
  * typos in driver names will be detected at compile time
  * avoids having several copies of the same string around
  * no error checking required when setting / getting value

The names used mirror those in the
virDomainHostdevSubsysPCIBackendType enumeration.