Remote backup of offline VM with multiple disks fails: Error during remote command

[abbbi]

Version used
1.9.3

Describe the bug
Remote backup of offline virtual machine with multiple disks fails:
[2022-11-15 21:54:19] CRITICAL virtnbdbackup - main [MainThread]: Unknown Exception during backup: Error during remote command: [qemu-nbd -r --format=qcow2 -x sda /tmp/tmp.J57ASJL2Dq/vm3-sda.qcow2 -p 10810 --pid-file /tmp/tmp.J57ASJL2Dq/qemu-nbd-backuppojmh4o2.pid --fork > /tmp/tmp.J57ASJL2Dq/qemu-nbd-backuphrx0myyv.log 2>&1]: []
[2022-11-15 21:54:19] ERROR virtnbdbackup - main [MainThread]: Error during remote command: [qemu-nbd -r --format=qcow2 -x sda /tmp/tmp.J57ASJL2Dq/vm3-sda.qcow2 -p 10810 --pid-file /tmp/tmp.J57ASJL2Dq/qemu-nbd-backuppojmh4o2.pid --fork > /tmp/tmp.J57ASJL2Dq/qemu-nbd-backuphrx0myyv.log 2>&1]: []
Traceback (most recent call last):

Cause

If backup runs with multiple workers, the used remote port for the offline NBD service is not unique and defaults to the default
ndb port. Multiple processes attempt to share the same port.

Workaround:

Limit amount of workers