Revert "Implement set_owner/get_owner using the libvirt API directly + fix get_diskspace failure breaking the whole list_vms in some cases (TUI-94, TUI-95)"

This reverts commit 3c13ac470a4c27765e41ad0f87dc9b6192bd4886.
Implement KVM set_owner/get_owner using ElementTree and raw file writing (OMS-515)