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>
