Write unit-test cases for scope/render/persistentvolume.go.

[avishnu]

FEATURE REQUEST:

• Write unit-test cases for the file: scope/render/persistentvolume.go.
• The test-cases have to be written in scope/render/persistentvolume_test.go.
• Ensure that the test cases are not too complicated. Follow the guidelines https://github.com/openebs/openebs/blob/master/contribute/CONTRIBUTING-TO-DEVELOPER-DOC.md