clarify autogenerated status of main.ml
improve xen support

* do not clash the object output name with one of the ML modules,
as ocamlopt also uses the `.o` suffix.  Instead, rename our target
excutable to `mir-<name>` and use that for the final target.
* force symlink construction via `ln -nfs`
* intermediate build errors with xen are a hard failure now.
check more carefully for a missing mirage-fs, and output useful error msg
Remove the need for `mir-run` by including the final Xen link directly in Mirari.
add CHANGES
install mirage-fs automatically instead of erroring out