libxl: detect support for save and restore
    
    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