Forward chr_add_watch call from mux chardev to underlying
implementation.

This should fix bug #1335444

Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>