virt-manager is GUI for running virtual machines in qemu similar to virtual box and vmware. The virtual machines are independent of the virt-manager GUI, so closing the GUI leaves the VM running in background.

Install virt-manager with apt:

apt-get install virt-manager
# add user to libvirt group
usermod -aG libvirt USER

When the user is not added to the libvirt group, virt-manager starts with an error:

Unable to connect to libvirt qemu system

journalctl -u libvirtd.service
libvirtd  authentication unavailable no polkit agent available to authenticate action org.libvirt.unix.manage

On some machines, the hw virtualization is disabled, it can be check with this command:

virt-host-validate
  QEMU: Checking for hardware virtualization                                 : FAIL (Only emulated CPUs are available, performance will be significantly limited)

Enable hw virtualization in bios.

Problem after reboot, can't start VM because of error: Network 'default' is not active

Run commands to activate the default network in libvirt:

> virsh net-list --all
 Name   State   Autostart   Persistent
----------------------------------------

# if the list is empty, define the default network
virsh net-define /usr/share/libvirt/networks/default.xml

virsh net-autostart default

# start the default network
sudo virsh net-start default