VMware store: Use the Content-Length if available

Change I579084460e7f61ab4042632d17ec0f045fa6f5af changed the VMware
store to use chunked encoding to upload data to the underlying backend.
We should do that only if the image size is not provided or zero and not
all the time.

This patch addressed the issue by checking the image_size before upload.

Change-Id: If348be7fd24fd05146b0c86bef45b79f600cc0f7
Closes-Bug: #1336970