virnuma: Check for numa_bitmask_isbitset presence

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>