storage: dir: .buildVol and .buildVolFrom callbacks for ploop
These callbacks let us to create ploop volumes in dir, fs and etc. pools.
If a ploop volume was created via buildVol callback, then this volume
is an empty ploop device with DiskDescriptor.xml.
If the volume was created via .buildFrom - then its content is similar to
input volume content.
Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com> Signed-off-by: Ján Tomko <jtomko@redhat.com>