snapshotctl: Check device for -img path

If the device exists only on /data, then the image path should
be checked with -img.

Bug: 330744468
Test: snapshotctl apply-update
Change-Id: I64ff7d803fdaaa02d2d6929994ad1124bfffab8b
Signed-off-by: Akilesh Kailash <akailash@google.com>