The message is a notification from the hypervisor. It is telling you two things:
This is the most common productive use of a virtual serial port. You can redirect all output from the guest’s serial port to a file on the host. virtual device serial0 will start disconnected
The "serial0" disconnection warning is largely a symptom of legacy software deprecation. While it rarely impacts system stability, maintaining a clean configuration by removing unsupported devices ensures smoother VM migration and consistent snapshot performance. The message is a notification from the hypervisor
In an era of USB-C and Thunderbolt, why do modern virtualization tools still emulate serial ports by default? virtual device serial0 will start disconnected