util: Make virStringArrayHasString() const-correct
    
    The first argument should be const char ** instead of
    char **, because this is a search function and as such it
    doesn't, and shouldn't, alter the haystack in any way.
    
    This change means we no longer have to cast arrays of
    immutable strings to arrays of mutable strings; we still
    have to do the opposite, though, but that's reasonable.