Commit 80533ca2 forgot to think about offline cpus.  When a node
cpu is offline, then its topology/ subdirectory is not present,
leading to spurious error messages leaked to the user such as:

libvir:  error : cannot open /home/dummy/libvirt/tests/nodeinfodata/linux-nodeinfo-sysfs-test-6/node/node0/cpu7/topology/physical_package_id: No such file or directory

Fix that, as well as test it; the test data is gathered from a
machine with one NUMA node, hyperthreading, and with 2 of the
8 cpus offline.

* src/nodeinfo.c (virNodeParseNode): Don't parse topology of
offline cpus.
* tests/nodeinfotest.c (mymain): Run new test.
* tests/nodeinfodata/linux-nodeinfo-sysfs-test-6*: New data.