Correct InvalidAggregateAction reason for Xen

Currently the xenapi will report InvalidAggregateAction
when add aggreate under some conditions. But the reason
didn't use the pre-defined reason list.

Change-Id: I2cd65f26097f154f1a4f794916c74cd1730ae142
Move check_image_exists out of try in _inject_data

Move check_image_exists out of try in _inject_data and some typo fixes

Change-Id: I9a6de0f993222c1921a0fc4d13ec14d875af2ed1
Merge "Move check_image_exists out of try in _inject_data"
Merge "Correct InvalidAggregateAction reason for Xen"