xen-unstable c/s 20762 bumped XEN_SYSCTL_INTERFACE_VERSION to 7.  The
interface change does not affect libvirt, other than xenHypervisorInit()
failing since version 7 is not tried.

The attached patch accommodates the upcoming Xen 4.0 release by checking
for XEN_SYSCTL_INTERFACE_VERSION 7.  If found, it sets
XEN_DOMCTL_INTERFACE_VERSION to 6, which is also new to Xen 4.0.