I had asked same question on Xen forums as well. Here is the summary:
These API functions were exported by 5da779c3 ("mm: export use_mm/unuse_mm to modules") added in 2.6.34. The current XenServer kernel is based on 2.6.32 and is therefore not new enough to have this change.
XenServer will be moving to a 3.x dom0 kernel in the next major release.