QEMU handles "address already in use" error, and successifully rebinds to socket, which is left in the filesystem by previous invocation.

Socket chardev should be created with nowait option, otherwise QEMU would wait until somebody connects to the socket before demonizing.

Correct options:

-chardev socket,server,nowait,path=/home/vitus/asta.monitor,id=monitor \
-mon chardev=monitor,mode=readline \