Fix bug in iallocator data structures build

Commit a1cef11c fixed non-vm_capable nodes export, but broke
inadvertently offline nodes. The update of the dict only needs to
happen for online nodes, in the 'if' block.

Without this patch, offline nodes keep the data from the last node
that was not offline; end result is that all nodes are considered
online (unless the first node is offline, in which case an error will
be raised).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>