virtual-device: Return empty no-match if unknown SCAN id is passed

This matches the expectation. i.e. we return no-match and we do not return a scanned print as we don't have anything for it. If we did indeed return a scanned print, then fprintd would try to delete it during enroll and would then fail.

Note that we do not return a DATA_NOT_FOUND error in the storage device if the print does not exist. This is because not all devices support reporting this error. It is therefore more sensible to handle it gracefully and expect test setups to set the error explicitly for testing purposes.