Unfortunately this requires a bit of trickery with pkg-config to get to
work. The root issue is that the mirage-xen assumes that we use the
default libdir of dune ($out/lib) whereas we install to an OCaml
version-specific site-lib directory. Thus the manually created
pkg-config file makes wrong assumptions (which warrants a patch) and the
.pc file is installed to the wrong location (which is fixed with a mv
invocation).
