nodeinfo: support kernels that lack socket information

nodeinfo: support kernels that lack socket information

On RHEL 5, I was getting a segfault trying to start libvirtd,
because we were failing virNodeParseSocket but not checking
for errors, and then calling CPU_SET(-1, &sock_map) as a result.
But if you don't have a topology/physical_package_id file,
then you can just assume that the cpu belongs to socket 0.

* src/nodeinfo.c (virNodeGetCpuValue): Change bool into
default_value.
(virNodeParseSocket): Allow for default value when file is missing,
different from fatal error on reading file.
(virNodeParseNode): Update call sites to fail on error.