monitor: protect outbuf and mux_out with mutex This lets the block layer emit QMP events from outside the I/O thread. Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>