generic/459: work with 64k block size

generic/459: work with 64k block size

With 64k block size, 200MiB disk space is not sufficient to create an
XFS filesystem. Hence this commit increases the size of the
overprovisioned dm-thin device to 300MiB. The commit also increases the
other associated disk sizes (original physical size and new physical
size) appropriately.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Tested-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>