[FEATURE] Enable backing image feature in Longhorn

[yasker]

Longhorn Engine has the ability to use a qcow2 file as the backing image for the Longhorn volume. It was designed for VM but the feature was never implemented in Longhorn manager nor UI. We need to enable this feature for Harvester to use Longhorn with existing root images.

Btw, the field was called base image but it's better to rename everything into backing image because base image has become a standard Docker term.

Ref:
#1989 (comment)
harvester/harvester#227

[longhorn-io-github-bot]

Pre-merged Checklist

• Is the reproduce steps/test steps documented?

• Is there a workaround for the issue? If so, is it documented?

• Does the PR include the explanation for the fix or the feature?

• Is the backend code merged (Manager, Engine, Instance Manager, BackupStore etc)?
  The PR is at
  chart: Update chart for the backing image feature #2229
  Introduce Backing image longhorn-manager#817
  Enable Backing image longhorn-engine#557
  Store and present backing image info for backup volumes backupstore#60

• Which areas/issues this PR might have potential impacts on?
  Area Volume attachment, detachment, expansion, RW, backup, restore
  Issues

• If labeled: require/LEP Has the Longhorn Enhancement Proposal PR submitted?
  The LEP PR is at enhancement: Add a new enhancement 'enable backing image' #2133

• If labeled: area/ui Has the UI issue filed or ready to be merged?
  The UI issue/PR is at Support raw or qcow2 file as the backing image longhorn-ui#354

• if labeled: require/doc Has the necessary document PR submitted or merged?
  The Doc issue/PR is at Docs for the new feature Backing image  website#259

• If labeled: require/automation-e2e Has the end-to-end test plan been merged? Have QAs agreed on the automation test case?
  The automation skeleton PR is at Add tests for Backing Image feature longhorn-tests#524
  The automation test case PR is at Add tests for Backing Image feature longhorn-tests#524

• if labeled: require/automation-engine Has the engine integration test been merged?
  The engine automation PR is at Enable Backing image longhorn-engine#557

• if labeled: require/manual-test-plan Has the manual test plan been documented?
  The updated manual test plan is at Add tests for Backing Image feature longhorn-tests#524

• If the fix introduces the code for backward compatibility Has an separate issue been filed with the label release/obsolete-compatibility?
  The compatibility issue is filed at

[yasker]

Move to Icebox. Pending merge of #2295

[innobead]

The new version of backing image implementation is actually be based on #2295 . To avoid confusing QA for these similar tickets, close this and move focus on #2295.