CF-574 VM migration when backing file is not present
CF-574 VM migration when backing file is not present

VM migration fails if backing file for the ephemeral drive is somehow
missing in source cloud. The patch resolves this problem by ignoring
missing backing file and only copying the base ephemeral.

MirantisWorkloadMobility/CloudFerry,MirantisWorkloadMobility/CloudFerry,SVilgelm/CloudFerry,SVilgelm/CloudFerry,SVilgelm/CloudFerry,SVilgelm/CloudFerry,MirantisWorkloadMobility/CloudFerry,MirantisWorkloadMobility/CloudFerry