Improved HTTP and FAT filesystem support

• Do not remove OPAM packages when doing mirage clean (#143)
• [xen] generate a simple main.xl, without block devices or network interfaces.
• The HTTP dependency now also installs mirage-tcp-* and mirage-http-*.
• Fix generated Makefile dependency on source OCaml files to rebuild reliably.
• Support Fat_KV_RO (a read-only k/v version of the FAT filesystem).
• The Unix KV_RO now passes through to the underlying filesystem instead of calling crunch, via mirage-fs-unix.