tests: Adjust libxlxml2domconfigtest to work with Xen < 4.8
tests: Adjust libxlxml2domconfigtest to work with Xen < 4.8

Commit fcdc387410fadfb066b95395c5b5d2a6a16f7066 used a libxl API which
is only available since Xen 4.8.

Due to lack of a specific guard for this API change, reuse another
guard from libxl.h.

Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>
Reviewed-by: Jim Fehlig <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>

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