build: avoid obsolete index()
Commit 2a530a3e5 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: Eric Blake <eblake@redhat.com>