fixed disk identifiers

This is useful for [Reproducible Builds](https://wiki.debian.org/ReproducibleBuilds) / [ Verifiable Builds ](https://www.whonix.org/wiki/Verifiable_Builds).

This change does nothing by default. Otherwise, once the `FIXEDDISKIDENTIFIERS` environment variable is set to `yes`, it will change the disk signature and uuid of the vmfile to fixed values.