Libvirt: Add backup

Libvirt: Add backup

The directory /etc/libvirt is backed up on uninstallation  and is
restored on installation.

Alle Files in  /var are commented in the rootfile so they are not
removed on uninstallation.
Because of the fact that the directories are not shipped with the
package they were created at installation time.
The permissions of 3 directories are changed because the qemu user is
nobody and the qemu group is kvm, so the permissions must be nobody:kvm

Fixes: #11151
Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>