fix(localpv): restore size to return as part of snapshotshot create response

[hrudaya21]

Pull Request template

Please, go through these steps before you submit a PR.

Why is this PR required? What issue does it fix?:
#416

What this PR does?:
return extra size info when return CreateSnapshot response

Does this PR require any upgrade changes?:
I think normal flow doesn't change, only affect external repo, like kubevirt depends on it to create PVC success.

If the changes in this PR are manually verified, list down the scenarios covered::
After this fix, the snapshot should have non-zero restore size.

Any additional information for your reviewer? :
Mention if this PR is part of any design or a continuation of previous PRs

Checklist:

• Fixes volumesnapshot restoreSize is not set when returning from createSnapshot response #416
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[sinhaashish]

lgtm