libxl: don't break the build on Xen>=4.5 because of libxl_vcpu_setaffinity()
    
    libxl interface for vcpu pinning is changing in Xen 4.5. Basically,
    libxl_set_vcpuaffinity() now wants one more parameter. That is
    representative of 'VCPU soft affinity', which libvirt does not use.
    
    To mark such change, the macro LIBXL_HAVE_VCPUINFO_SOFT_AFFINITY is
    defined. Use it as a gate and, if present, re-#define the calls from
    the old to the new interface, to avoid breaking the build.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
    Cc: Jim Fehlig <jfehlig@suse.com>
    Cc: Ian Campbell <Ian.Campbell@citrix.com>
    Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>