After an automatic update of CentOS to version 7.7 on Sep 17 2019 my QEMU/KVM virtual machines do not start when I reboot the hypervisor server.

Trying to start the VM by hand gives this error:

# virsh start mygreatvm
error: failed to connect to the hypervisor
error: no connection driver available for <null>

Trying to list currently defined VMs (the VMs in questions are set to start by default (autostart) on every boot so they should at least appear here):

# virsh list
error: failed to connect to the hypervisor
error: no connection driver available for <null>

There are no error messages in system logs and neither in /var/log/libvirt/qemu/*.log. The libvirt daemon seems to run fine and does not complain about the non-started servers in /etc/libvirt/qemu/autostart/*xml:

# systemctl status libvirtd
● libvirtd.service - Virtualization daemon
   Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2019-09-18 16:42:30 UTC; 2min 49s ago
     Docs: man:libvirtd(8)
 Main PID: 1817 (libvirtd)
    Tasks: 16 (limit: 32768)
   CGroup: /system.slice/libvirtd.service
           └─1817 /usr/sbin/libvirtd
Sep 18 16:42:30 server systemd[1]: Starting Virtualization daemon...
Sep 18 16:42:30 server systemd[1]: Started Virtualization daemon.

The updates in CentOS 7.7 include an update to QEMU which needs a new package to be able to start QEMU/KVM virtual machines.

# yum install libvirt-daemon-driver-qemu

Then the virtual machines can be started right away (no reboot necessary):

# systemctl restart libvirtd
# virsh list
 Id    Name                           State
 1     mygreatvm                      running
 2     mygreatvm2                     running
 3     mygreatvm3                     running

Some CentOS hypervisors might have this package already installed; there will be no failures on these machines but the package was not necessary in the past.

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