Xenapi: Don't resize down if not auto_disk_config
Xenapi: Don't resize down if not auto_disk_config

It is not safe to resize down an instance if the filesystem has not been
resized first, but with auto_disk_config == False there's no guarantee
that it has happened.  And not resizing down the disk space with the
instance allows for users to have more space on their instance than the
type should allow.

Fixes bug 1081225

Change-Id: I980c0699dfc272155e274a96d6e08e131c1372d9

n0ano/ganttclient