Use an abstract type Vchan.Port.t instead of a string

This is another backwards-incompatible change. This prevents the user
accidentally providing an invalid port (which must be a valid xenstore
path element)

Signed-off-by: David Scott <dave.scott@citrix.com>
Bump version to 2.0.0 (backwards incompatible) and update CHANGES

Signed-off-by: David Scott <dave.scott@citrix.com>
Regenerate OASIS

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