nodeinfo: skip offline CPUs

nodeinfo: skip offline CPUs

https://bugzilla.redhat.com/622515 - When hot-unplugging CPUs,
libvirt failed to start a guest that had been pinned to CPUs that
were still online.

Tested on a dual-core laptop, where I also discovered that, per
http://www.cyberciti.biz/files/linux-kernel/Documentation/cpu-hotplug.txt,
/sys/devices/system/cpu/cpu0/online does not exist on systems where it
cannot be hot-unplugged.

* src/nodeinfo.c (linuxNodeInfoCPUPopulate): Ignore CPUs that are
currently offline.  Detect readdir failure.
(parse_socket): Move guts...
(get_cpu_value): ...to new function, shared with...
(cpu_online): New function.