You are trying to mount and connect to an image using qemu-nbd:
qemu-nbd -c /dev/nbd0 rtt.qcow2
qemu-nbd: Failed to set NBD socket
qemu-nbd: Disconnect client, due to: Failed to read request: Unexpected end-of-file before all bytes were read
Another related error:
qemu-nbd: Failed to unlink socket /var/lock/qemu-nbd-nbd0: Operation not permitted
Error with nbd, exiting........