Making getVSize and getVTrueSize SD methods.

* Units sanitized. The two methods return now size in bytes,
  like st_size.
* getVTrueSize() renamed to getAllocSize().

TODO:
These two method should be unified in a future patch since the
two values are returned in the same call for file volumes and
the two values are identical for block volumes.

Required to reduce the volume size functions proliferation.

Change-Id: I3d9772cedd74431e71d6068399546e1b4aae69e9
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18202
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>