1. Moved the deletion of the tree to after the image file is chosen.  2. strategy to choose sector_size

...s

will not delete the existing tree in case no image file is chosen.
2. Various scenarios to pick sector_size implemented. If sector size is not
present in dfxml file, the secctor size from 1st volume is chosen. If 1st
volume itself doesn't have it, it is defaulted to 512 bytes.