Clarify note on QCOW2 uploads

Reworded paragraph on converting images to QCOW2 before upload, since
it might be faster to upload a smaller QCOW2 image instead of a RAW image.

Change-Id: I32fc57d2dda36c5d3c1f5d21aaaaa2a8044cd0a2
Closes-Bug: 1384011