parallels: add disks correctly
    
    Disks support in this driver was implemented with an assumption,
    that disk images can't be created by hand, without VM. So
    complex storage driver was implemented with workaround.
    
    This is not true, we can create new disks using ploop tool.
    So the first step to reimplement disks support in parallels
    driver is to do not use information from the storage driver,
    until we will implement VIR_STORAGE_TYPE_VOLUME disks.
    
    So after this patch disks can be added in the same way as
    in any other driver: you create a disk image and then add
    an entry to the XML definition of the domain with path to that
    image file, for example:
    
    <disk type='file' device='disk'>
      <driver type='ploop'/>
      <source file='/storage/harddisk1.hdd'/>
      <target dev='sda' bus='sata'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    
    This patch makes parallels storage driver useless, but I'll fix it
    later. Now you can create an image by hand, using ploop tool,
    and then add it to some domain.
    
    Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>