feat: a new VM's CPU model can be chosen, from a list the hypervisor offers

create_vm_from_cloud_init takes cpu_type. Proxmox gives a VM created without
one the kvm64 model, which has no AVX, so MongoDB 5.0 and later do not start
there, and netOrk's graylog role failed on every VM it provisioned
(netork#494). Which model is right depends on the cluster: host cannot
live-migrate between different CPUs, x86-64-v3 does not start on a CPU older
than Haswell. So the caller chooses.

get_vm_cpu_types() is the new, optional listing behind that choice. Each
VMCpuTypeDict names the model, says what it is for, lists the /proc/cpuinfo
flags the guest gets (so a caller can ask "does this give AVX?" without
knowing model names), whether the node at hand can run it, and which one is
the default. A hypervisor whose VMs have no per-VM CPU model (VMware sets CPU
compatibility per cluster) does not implement it and must reject any
cpu_type other than None.

Both declarations sit under TYPE_CHECKING like the rest of the contract, so
hasattr stays a truthful capability probe; the tests read the signature from
the source.

README.md

@@ -247,6 +247,12 @@ class ProxmoxDriver(HypervisorDriver):
 
     def create_vm_snapshot(self, name, snapshot, description="", include_memory=False):
         ...
+
+    def get_vm_cpu_types(self):
+        # optional — return List[VMCpuTypeDict]: the CPU models a new VM may get
+        # on this node, each with its cpuinfo flags and whether the node can run
+        # it; the name goes to create_vm_from_cloud_init(cpu_type=...)
+        ...
 ```
 
 ### OS / Linux