char: update the mux handlers in class callback

Instead of handling mux chardev in a special way in
qemu_chr_fe_set_handlers(), we may use the chr_update_read_handler
class callback instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190206174328.9736-2-marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>