xenapi: add support for auto_disk_config=disabled

Previously, only True and False were supported. This change allows an
administrator to specify which images should reject any user requests
for DiskConfig=AUTO, i.e. reject the request to automatically resize
that instance's root disk.

To specify that an image's disk can't be resized, add the following
property to the image:
auto_disk_config=disabled

In addition, when uploading a snapshot we need to ensure that if
auto_disk_config is "disabled", that property is inherited by any
snapshot. Currently, the disk_config from the database, which is always
False in the case of "disabled", would be persisted instead. When
auto_disk_config is not "disabled", then the current True or False value
for that server should continue to be persisted in any snapshot, and not
the value stored in the source image for that server.

DocImpact
Implements blueprint auto-disk-config-disabled
Change-Id: Ifd41886b9bc7dff01cdf741a833946bed1bdddc3
Merge "xenapi: add support for auto_disk_config=disabled"