This adapts the `mirage` front-end tool to the new Mirage/Xen platform
stack (see mirage#1159).

User-visible changes:

- `.xe` (Xenserver) filegeneration has been removed, I have no way to
test this.
- "Unknown" argument filtering in `Bootvar` / `argv` has been removed,
this was causing problems and I do not know of any users apart from
Qubes OS where this can be solved differently, see comments in
mirage#1159.

Note that Xen unikernels now include a dummy Solo5 manifest to keep the
rest of the build system and Solo5 layers happy; this does not contain
any devices as they are not required and Xen device naming does not
follow Solo5 requirements.

Thus, tcpip.xen is not needed to be linked anymore.

…unds for the upcoming release

These two things are hardly related, keep them separate

mirage does not contain any tests, avoid unneccessary rebuilds