xenapi: cleanup vdi when disk too big exception raised

In addition add some unit tests to that area of the code.

fixes bug 1015423

Change-Id: If33cbb395bc39cbb6e8b2ecbd2d6ad3a26b6b301
Merge "xenapi: cleanup vdi when disk too big exception raised"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp