Revert "char: use a fixed idx for child muxed chr"

That commit mis-used mux char: the frontend are multiplexed, not the
backend. Fix the regression preventing "c-a c" to switch the focus. The
following patches will fix the crash (when leaving or removing frontend)
by tracking frontends with handler tags.

This reverts commit 949055a2549afc4cde06b7972072c7288bb43722.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
