Fix calculation of file volume allocated size

Allocated size was computed using st_blksize as block size. st_blksize
is the "preferred" buffer size for efficient file system I/O on the file
system where the block is. This patch use block size of 512 bytes as
specified in stat(2) manual.

Change-Id: I71646199e2c0ac4564b832b8ebb202f49e289344
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21490
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>