add value check for the vapp.update_product_section.

[devendrapratapsingh]

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There is an issue to update the product section(guest properties) in the VM using vapp.py

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documentation.
Old code was removing the existing property and then append again but without label and other fields.
There is a check required if key is already exist then update the value so that it'll be visible correctly and have the unique set of key in the product section. Old code was created same key multiple times.
detailed messages, refer to those instead

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This change is 

[rocknes]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devendrapratapsingh)