Fix xlconfigtest with older libxl
Fix xlconfigtest with older libxl

Commit cd5dc30 added this test, but it fails if
LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST is not defined:

6) Xen XM-2-XML Format fullvirt-multiusb
... libvirt:  error : unsupported configuration: multiple USB
devices not supported
FAILED

andreabolognani/libvirt,taget/libvirt,agx/libvirt,VenkatDatta/libvirt,taget/libvirt,rlaager/libvirt,taget/libvirt,jfehlig/libvirt,nertpinx/libvirt,datto/libvirt,VenkatDatta/libvirt,datto/libvirt,crobinso/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jardasgit/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,rlaager/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,olafhering/libvirt,agx/libvirt,agx/libvirt,crobinso/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,agx/libvirt,nertpinx/libvirt,olafhering/libvirt,VenkatDatta/libvirt,taget/libvirt,zippy2/libvirt,jfehlig/libvirt,taget/libvirt,nertpinx/libvirt,jfehlig/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,crobinso/libvirt,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt,zippy2/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,libvirt/libvirt,andreabolognani/libvirt,jfehlig/libvirt,rlaager/libvirt,rlaager/libvirt,eskultety/libvirt,eskultety/libvirt,VenkatDatta/libvirt,agx/libvirt,nertpinx/libvirt,eskultety/libvirt,datto/libvirt,rlaager/libvirt,datto/libvirt