nodeinfo: Increase the num of CPU thread siblings to a larger value
    
    Current libvirt can only handle up to 1023 bytes when it
    reads Linux sysfs topology/thread_siblings. This isn't enough for
    Linux distributions that support a large value. This patch fixes
    the problem by using VIR_ALLOC()/VIR_FREE(), instead of using a
    fixed-size (1024) local char array. In the meanwhile
    SYSFS_THREAD_SIBLINGS_LIST_LENGTH_MAX is increased to 8192 which
    should be large enough for a foreseeable future.
    
    Signed-off-by: Wei Huang <wei@redhat.com>