Commit 2a530a3e is not portable to mingw, which intentionally
avoids declaring the obsolete index().  See also:
https://bugzilla.redhat.com/show_bug.cgi?id=1214605

* src/util/virstring.c (virStringStripControlChars): Use strchr.
Signed-off-by: NEric Blake <eblake@redhat.com>