This commit is to deprecate the config of image_upload_handler
which need to be configured with a class path string. And we
should make download handler be configurable to be consistent
with the upload handler.
So it contains the following changes:
1. deprecate the option of "image_upload_handler".
2. added a new option of "image_handler" which will be used to
choose a short name from the supported image handler plugins.
The proper handler will be loaded at runtime basing on the
option setting.
TODO(jianghuaw): For the legacy handler of VdiThroughDevStore
which has no download function implemented. For downloading, it
is handled in a different way (call xenapi.vm_utils._stream_disk()).
We keep it as it is now. In the future, we should consider to
move _stream_disk to VdiThroughDevStore's download().
Change-Id: I37860e63de32d988525cdc32440fe711777263e6
Blueprint: xenapi-image-handler-option-improvement
238b89b36e