handle case where there missing are cpu details

1.  Look only at the <cpu> section in the virsh capabilities
       otherwise, it could pull info from the wrong section
2.  put try/except block around each lookup
       vendor, model, topology
3.  Print Unknown for each if we can't determine it from xml
4.  If we don't know the vendow, don't print anything for
       NX or EVP

rhbz#887149

Change-Id: I1811bb17bf48d66cf601f564fd6cb75af681e004
Signed-off-by: Mike Burns <mburns@redhat.com>