Check image exists before calling inject_data

os.stat removal is needed for network-based image backends

Partly revert Change-Id: I264540de736c2bcb92567826fe5ba672e1244ba2
Partly fixes bug #1257674

Change-Id: I83e95fec6f08bc36f0e2b299fb269ceaa3500243
Merge "Check image exists before calling inject_data"