Use the ppc64Driver prefix for all functions that are used to
fill in the cpuDriverPPC64 structure, ie. those that are going
to be called by the generic CPU code.

This makes it clear which functions are exported and which are
implementation details; it also gets rid of the ambiguity that
affected the ppc64DataFree() function which, despite what the
name suggested, was not related to ppc64DataCopy() and could
not be used to release the memory allocated for a
virCPUppc64Data* instance.

No functional changes.