libxl does not support save, restore, or migrate on all architectures,
notably ARM.  Detect whether libxl supports these operations using
LIBXL_HAVE_NO_SUSPEND_RESUME.  If not supported, drop advertisement of
<migration_features>.

Found by Ian Campbell while improving Xen's OSSTEST infrastructure

http://lists.xen.org/archives/html/xen-devel/2014-06/msg02171.html