virnuma: Implement virNumaGetDistances stub for non-NUMA
    
    In case the libvirt is built without numactl support, we're
    missing the virNumaGetDistances() stub so the linking fails:
    
      CCLD     libvirt_lxc
    libvirt_lxc-nodeinfo.o: In function `virNodeCapsGetSiblingInfo':
    /home/zippy/tmp/libvirt.git/src/nodeinfo.c:1763: undefined reference to `virNumaGetDistances'
    collect2: error: ld returned 1 exit status
    make[3]: *** [libvirt_lxc] Error 1
    
    The issue was introduced in 77c830d8c4.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>