vmx: Expose datacenter path in domain XML
    
    Tool such as libguestfs need the datacenter path to get access to disk
    images. The ESX driver knows the correct datacenter path, but this
    information cannot be accessed using libvirt API yet. Also, it cannot
    be deduced from the connection URI in a robust way.
    
    Expose the datacenter path in the domain XML as <vmware:datacenterpath>
    node similar to the way the <qemu:commandline> node works. The new node
    is ignored while parsing the domain XML. In contrast to <qemu:commandline>
    it is output only.