xencapstest: Don't fail when Xen is installed

xencapstest: Don't fail when Xen is installed

xencapstest calls xenHypervisorMakeCapabilitiesInternal with conn == NULL
which calls xenDaemonNodeGetTopology with conn == NULL when a recent
enough Xen was detected (sys_interface_version >= SYS_IFACE_MIN_VERS_NUMA).
But xenDaemonNodeGetTopology insists in having conn != NULL and fails,
because it expects to be able to talk to an actual xend.

We cannot do that in a 'make check' test. Therefore, only call the xend
subdriver function when conn isn't NULL.

Reported by Andy Howell and Jim Fehlig.