IAllocator: fix breakage for non-vm_capable nodes

Currently, the IAllocator doesn't know that non-vm_capable nodes
should not be queried for hypervisor/instance information.

This patch updates the node data computation to generate basic info,
but not the runtime data for such nodes. Furthermore, the
documentation is updated to note that such nodes will node have
runtime data.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>