mplayer: don't call libquvi for local files

This is obviously not needed, and just creates potential for bad
breakages (e.g. what happens if libquvi tries to open a normal filename
as http URL?).

Note that for simplicity, we still pass file:// URIs to quvi, and we
don't exclude other protocol prefixes either. In general, we don't know
what protocols quvi might support, so we don't try to second-guess it.
(Even though in practice, it's probably only "http" and "https".)