Add helper methods to convert disk

Add a _disk_raw_to_qcow2 and _disk_qcow2_raw methods which
convert disk.
Update finish_migration to use these helpers.

Change-Id: If98f7fc04668916fda788afb65116a89463b0020
Related-Bug: #1298975
Record right action name while migrate

Actions are recorded in database while users take atcions in specific
instance. These actions can be listed by comand 'nova instance-action-list'.
Resize and migrate use same code path, but always record the action as
'resize' even user migrate instance. This commit check the action
firstly then record right action.

Change-Id: I3462275fa3022173d55dfa0c87328d4bb518239b
Closes-Bug: #1323173
Merge "Record right action name while migrate"
Merge "Add helper methods to convert disk"