[pve-devel] Fix Bug #889 and allow automatic VMID selection on create

[Thomas Lamprecht]

The get /cluster/nextid API call is not secured against race conditions and
parallel accesses in general.
Users of the API which created multiple CT in parallel or at least very fast
after each other run into problems here: multiple calls got the same VMID
returned.

Fix this by allowing the /cluster/nextid call to virtually reserve VMIDs
temporary.
Also add the possibility that the create_vm calls from CTs and VMs can auto
select a VMID, for convenience (also it was requested as a feature).

This means changes on a few packages (cluster, common, container,
qemu-server)

The first to patches should be quite straight forward and relevant for the
bug fix.

The other three may be seen as RFC.