[xen]: (protocol extension) allow consoles to be given string labels

The label allows the VM to look up a console by label rather than by
numeric id.

This is considered experimental behaviour.

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

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #13 from djs55/add-cli

Experimental support for named consoles