node.xml: libvirt domain save folder is now in the biggest partition

libvirt saves virtual machine memory in /var/lib/libvirt/qemu/save
when the container is rebooted. /var partition can be too small
to hold all the RAM of all the VMs.
start host vm: when VM disk is not present properly create it

rocks start host vm did not handle properly the case of starting
a VM when the disk was not present. Some code refactoring