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
make the configuration file optional by scanning for `.conf` files in the current directory.

Now just `mirari build` works great, as long as there's one config file in the directory.
update README
Merge branch 'master' of github.com:avsm/mirari
Merge pull request #1 from avsm/master

working xen mirari