Ubuntu – KVM\qemu installation issue (18.10)

Update This solution is for LightDM (default display manager up to Ubuntu 16.04 LTS), not for GDM (default display manager in newer Ubuntu versions).

It's a bug

The bug is described and discussed here, in Ubuntu Launchpad.

Explanation

Shortly, there are four kind of users in Linux: root (UID 0), system users (low UIDs), regular users (high UIDs) and the nobody user (UID 65535, the last UID).

The Ubuntu login screen uses a service called Accountsservice (a lightdm component) that, by default, presents all the regular users in the login screen.

However, the current Libvirt/QEMU package for Ubuntu 16.04 uses a high UID for the user libvirt-qemu. It seems to be a bug (system packages usually creates system users, with low UIDs).

Workaround

Each regular user can also set custom options to it's login process (background image for the login screen, desktop environment - Gnome or KDE, etc). These options are saved in /var/lib/AccountsService/users.

Using these custom options, it's possible to mark a regular user as a system user to Accountsservice. So, Accountsservice will consider it a system user despite it's UID.

To do this to the libvirt-qemu user:

echo -e "[User]\nSystemAccount=true" > /var/lib/AccountsService/users/libvirt-qemu

After this, you should reset the Accountsservice cache:

service accounts-daemon restart

The previous commands should be run as root.

I had the same problem and for me the solution was to manually create the libvirtd group:

sudo addgroup libvirtd
sudo adduser YOURUSERNAME libvirtd

After that: virt-manager started without telling me to add myself to the libvirtd group anymore.