vmm: Add core scheduling support for vCPU threads

Add a core_scheduling option to --cpus with three modes of operation.
This feature takes advantage of a kernel feature that restricts
scheduling of processes on the SMT threads on the same core. This is
useful for mitigating certain classes of side-channel attacks and has
better performance that disabling SMT on the CPU.

- vm (default): All vCPU threads share one core scheduling cookie.
  They may be co-scheduled on SMT siblings while host threads are
  excluded - this has minimal performance impact and can even
  potentially improve performance from co-location.
- vcpu: Each vCPU gets a unique cookie preventing any two vCPUs from
  sharing SMT siblings. This has the strongest isolation but at some
  compromise of performance.
- off: No core scheduling applied (old behaviour).

This isolation is done by the kernel maintaining a "cookie" - threads
with the same cookie can share the same core.

In vCPU mode each vCPU thread the cookie is created when the thread
starts and each gets a unique cookie. For VM mode the first vCPU thread
(the leader) will create the cookie. All other vCPU threads started (via
hotplug or during boot) will have that cookie shared to it.

EINVAL/ENODEV from prctl is silently ignored so this works transparently
on kernels older than 5.14 that lack PR_SCHED_CORE or when SMT disabled.

Full details of this kernel feature can be found at:
https://docs.kernel.org/admin-guide/hw-vuln/core-scheduling.html

This implementation was inspired by crosvm's implementation - in
particular the enable_core_scheduling() function.

This is challenging to test via integration testing but the logging of
the received cookie shows it working:

VM case:

cloud-hypervisor:   0.243102s: <vcpu1> INFO:vmm/src/cpu.rs:1247 -- vCPU 1: core scheduling cookie = 0x33e4c167
cloud-hypervisor:   0.243102s: <vcpu0> INFO:vmm/src/cpu.rs:1247 -- vCPU 0: core scheduling cookie = 0x33e4c167

vCPU case:

cloud-hypervisor:   0.089356s: <vcpu0> INFO:vmm/src/cpu.rs:1247 -- vCPU 0: core scheduling cookie = 0x13993ad6
cloud-hypervisor:   0.089380s: <vcpu1> INFO:vmm/src/cpu.rs:1247 -- vCPU 1: core scheduling cookie = 0xd48e86e

Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -738,6 +738,10 @@ components:
             $ref: "#/components/schemas/CpuAffinity"
         features:
           $ref: "#/components/schemas/CpuFeatures"
+        core_scheduling:
+          type: string
+          enum: ["Vm", "Vcpu", "Off"]
+          default: "Vm"
 
     PciSegmentConfig:
       required: