[BACKPORT][v1.6.2][IMPROVEMENT] Expose virtual size of qcow2 backing images

[github-actions]

backport #7923

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

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

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at: crd: Add virtual size to backing images (backport #8267) #8313
  The PR for the chart change is at: crd: Add virtual size to backing images (backport #8267) #8313

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at
  Expose virtual size of qcow2 backing images (backport #208) backing-image-manager#221
  Expose virtual size of backing images (backport #2680) longhorn-manager#2723

• Which areas/issues this PR might have potential impacts on?
  Area: Backing Image

• If labeled: area/ui Has the UI issue filed or ready to be merged (including backport-needed/*)?
  The UI issue/PR is at

• If labeled: require/manual-test-plan Has the manual test plan been documented?
  The updated manual test plan is at

1. Create a raw backing image and run kubectl -n longhorn-system get lhbi <backing image name>. Then varify if Size == VirtualSize
2. Create a qcow2 backing image and run kubectl -n longhorn-system get lhbi <backing image name>. Then varify if Size can be different from VirtualSize

[chriscchien]

Verified pass on longhorn v1.6.x (longhorn-manager e6d5aa, backing-image-manager e39fa2) with test steps

Create backing images

• qcow2 from https://longhorn-backing-image.s3-us-west-1.amazonaws.com/parrot.qcow2
• raw from https://longhorn-backing-image.s3-us-west-1.amazonaws.com/parrot.raw

Thw raw backing image's size equal to virtualsize

> kubectl -n longhorn-system get lhbi raw -o yaml | grep size -A2
  size: 33554432
  uuid: 6f790739
  virtualSize: 33554432

Thw qcow2 backing image's size not equal to virtualsize

> kubectl -n longhorn-system get lhbi qcow2 -o yaml | grep size -A2
  size: 1161728
  uuid: 69bb5887
  virtualSize: 33554432