libxl: remove conditionals from discard configuration
libxl: remove conditionals from discard configuration

LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE exists since Xen 4.5.0

Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>
Reviewed-by: Jim Fehlig <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>

crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt