qemu-char: do not call chr_write directly

Make the mux always go through qemu_chr_fe_write, so that we'll get
the mutex for the underlying chardev.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>