Add initial support for the console xenstore connection protocol

Includes a simple test CLI which should be able to attach consoles.

Signed-off-by: David Scott <dave.scott@citrix.com>
update CHANGES

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: add xenstore_transport

Signed-off-by: David Scott <dave.scott@citrix.com>
Add a console backend to the library and CLI

Connect a console to a VM like this:

```
[root@st30 ~]# ./mirage-console connect trusty
Operating on VM domain id: 19
Creating device 1 (linux device /dev/tty1)
{ ref = 128; event_channel = 13 }
```

Then inside the guest:

```
[root@trusty ~]# cat > /dev/hvc1
hello
there
```

And observe in dom0:

```
hello
there
```

Then hit Control+C and it all cleans up.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #11 from djs55/add-cli

Add a simple CLI to attach consoles