Fix vdi_info.snapshot_of for SMAPIv1

SMAPI snapshot_of should be the parent's uuid, not its database
reference.

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>