Thank you, Aristarkh Zagorodnikov, for taking the time to file a bug report.
In addition to 18.04, does this feature similarly misbehave with newer versions of libvirt/qemu? If it's fixed in a newer version then perhaps there is a patch that can be backported. If it still isn't working properly in the latest versions, then it'd likely be worth reporting upstream as well.
Since there is not enough information in your report to begin triage or to differentiate between a local configuration problem and a bug in Ubuntu, I am marking this bug as 'Incomplete'. We would be grateful if you would: provide a more complete description of the problem, explain why you believe this is a bug in Ubuntu rather than a problem specific to your system, and then change the bug status back to 'New'.
Thank you, Aristarkh Zagorodnikov, for taking the time to file a bug report.
In addition to 18.04, does this feature similarly misbehave with newer versions of libvirt/qemu? If it's fixed in a newer version then perhaps there is a patch that can be backported. If it still isn't working properly in the latest versions, then it'd likely be worth reporting upstream as well.
Since there is not enough information in your report to begin triage or to differentiate between a local configuration problem and a bug in Ubuntu, I am marking this bug as 'Incomplete'. We would be grateful if you would: provide a more complete description of the problem, explain why you believe this is a bug in Ubuntu rather than a problem specific to your system, and then change the bug status back to 'New'.