Commits 5dbee5ea and cce46164

 fix disk upgrades concerning params
slot. Since 2.7 params slot should be empty and gets filled
any time needed.

Still ext template allows passing arbitrary params per disk.
These params should be saved in config file for future use.
For instance if we have the shared-filer provider and we
specify shared_dir param during instance create, this param
is needed when we want to attach the disk e.g., during
retrieving instance info. If it gets overridden during a daemon
restart or a config reload we fail to get the instance's info.

To avoid such a failure, we set params slot to an empty dict
only if params not found in the first place.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>