When formatting a qcow2 image with an external (raw) data file, we shouldn’t need to write to it except for data preallocation (falloc/full), if requested.

Taking the WRITE permission prevents simultaneous use of an existing raw data file (as a raw image) by a party that does not allow concurrent writes (i.e. unshares the WRITE permission), for example a guest device that has not been configured with share-rw=on. As a result, it is currently impossible to create a qcow2 “metadata overlay” for an existing raw image that is already in use by a VM.

To resolve the issue, during formatting, qcow2 should not take the WRITE permission for the external data file unless required for falloc/full preallocation.