virnuma: Check for numa_bitmask_isbitset presence
    
    On some systems, libnuma can be present but it's so ancient that
    it misses some symbols that virNumaGetDistances() needs. To be
    more precise: numa_bitmask_isbitset() and numa_nodes_ptr are the
    symbols in question. Fortunately, they were both introduced in
    the same release so it's sufficient for us to check for only one
    of them. And the winner is numa_bitmask_isbitset().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>