libxl: builtin list types should be pass-by-reference

libxl: builtin list types should be pass-by-reference

This makes all _destroy functions consistent wrt freeing the actual
reference passed in. Previously we were relying on the reference
contained within the type itself which worked but was semantically a
little confusing.

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