Add the CONSOLE signature to module S

This commit adds a S.CONSOLE signature which is implemented
for both Unix and Xen. This signature simply includes the
V1_LWT.CONSOLE standard one, but we can now add extra functions
which will eventually be upstreamed to mirage/types.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add support for reading from consoles, so we can be interactive

Signed-off-by: David Scott <dave.scott@citrix.com>
mirage-console: add a 'plug' subcommand to introduce a frontend to a backend

This allows us to hotplug consoles when we aren't implementing the
backend ourselves. Other backend providers are available: have
a look at xenconsoled and qemu-xen/qemu-system-*

Signed-off-by: David Scott <dave.scott@citrix.com>
Always build the mirage-console shared library

Although it needs xenstore (pure OCaml), it doesn't need xen-gnt
(C lib interface)

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #15 from djs55/add-cli

Add support for reading from consoles