libxl: include domain id in userdata path.

libxl: include domain id in userdata path.

The userdata is specific to a particular incarnation of a domain and
the patch is therefor required to be unique to each incarnation. If
the user has explicitly configured a UUID in their domain
configuration then the path is no longer unique since
22124:22366e13f76d "xl: randomly generate UUIDs" which (correctly)
caused the uuid domain configuration option to be obeyed.

If userdata is not unique to each incarnation of a domain then
localhost live migration is broken because the target is created (and
writes its userdata) before the sender destroys the domain (and
deletes its userdata).

Strictly speaking I think the UUID is unnecessary but it is perhaps
helpful to people looking in the userdata directory, for debugging
etc.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>