Revert Storage_interface.query_result.features to a (string list)

This means storage_interface.ml is now identical to the Tampa version.
The features are parsed into (string, int64) pairs at the xapi level
instead of at the SMAPI level.

Signed-off-by: John Else <john.else@citrix.com>
Re-use some code, and fix lookup of content_id in vdi_info_from_db

Signed-off-by: John Else <john.else@citrix.com>
Fix vdi_info.snapshot_of for SMAPIv1

Signed-off-by: John Else <john.else@citrix.com>
Add snapshot setters to SMAPI

SMAPI now includes:

VDI.set_is_a_snapshot
VDI.set_snapshot_time
VDI.set_snapshot_of

For SMAPIv1, we just set the new values in xapi's database. For SMAPIv2,
these fields will be implemented by the backend.

Signed-off-by: John Else <john.else@citrix.com>
Remove some commented-out code

This has been commented out since the api.hg days...

Signed-off-by: John Else <john.else@citrix.com>