parallels: create container from existing image
    
    It's possible to create a container with existing
    disk image as root filesystem. You need to remove
    existing disks from PCS VM config and then add a new
    one, pointing to your image. And then call PrlVm_RegEx
    with PRNVM_PRESERVE_DISK flag.
    
    With this patch you can create such container with
    something like this for new domain XML config:
    
        <filesystem type='file' accessmode='passthrough'>
          <driver type='ploop' format='ploop'/>
          <source file='/path-to-image'/>
          <target dir='/'/>
        </filesystem>
    
    Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>