util: Make virStringArrayHasString() const-correct

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.