xenapi: Resize operations could be faster

 * Improves the code by using only one time resize2fs during the resize.
 * Removes the not used method: "_get_min_sectors".
 * Uses a VDI clone instead of copy in resize_disk's method.
 * Review of some test cases to accept the new behavior
   without _get_min_sectors.

Closes-Bug: 1221332
Change-Id: I40a1d7d467353ba904689d128de7f89d785b4052
Merge "xenapi: Resize operations could be faster"