virnuma: Introduce virNumaNodeIsAvailable

virnuma: Introduce virNumaNodeIsAvailable

Not on all hosts the set of NUMA nodes IDs is continuous. This is
critical, because our code currently assumes the set doesn't contain
holes. For instance in nodeGetFreeMemory() we can see the following
pattern:

    if ((max_node = virNumaGetMaxNode()) < 0)
        return 0;

    for (n = 0; n <= max_node; n++) {
        ...
    }

while it should be something like this:

    if ((max_node = virNumaGetMaxNode()) < 0)
        return 0;

    for (n = 0; n <= max_node; n++) {
        if (!virNumaNodeIsAvailable(n))
            continue;
        ...
    }

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>