Install the xen/unix packages as a single META subtree

It is now mirage-console.[xen/unix] instead of mirage-console-unix
and mirage-console-xen.  This also fixes the uninstallation bug where
two opam packages both install mirage-console.
Do not depend on xenstore for the main mirage-console.

Instead, add `mirage-console.proto` which contains the Xen-specific
protocol setup code.  This way, mirage-console.unix doesn't depend
on Xenstore
simplify travis to use pins
add a local opam file for pinning
refine travi packages
opam: fix typo
travis: only test on opam 1.2.0 due to local pin feature
further simplifications of the Oasis subpackages and opam files
opam: add ipaddr dep to get mirage-types.lwt
Also install xenstore_transport for the CLI
Yet more opam fixes
build mirage-console.proto more often
Merge pull request #24 from avsm/unify-xen-unix

Simplify the mirage-console packaging