vmm: keep virtio activation alive in migration

[Coffeeri]

This fixes https://github.com/cobaltcore-dev/cobaltcore/issues/411

Live migration can deadlock if the guest triggers a virtio device
activation while the migration worker owns the VM.

The failure shows up during boot and firmware, where the guest can
reset and reinitialize virtio devices while precopy is running. In the
failing case, the source log shows a pending virtio activation that
never completes:

  8.115833s _virtio-pci-net_0: Needs activation; returning barrier
  8.115854s vmm/src/vm.rs:464 -- Waiting for barrier
  24.875452s Entering downtime phase
  24.875481s stopping vcpu throttling thread
  ...
  vCPU thread did not respond in 10ms to signal - retrying
  vCPU thread did not respond in 20ms to signal - retrying
  ...
  thread 'throttle-vcpu' (1029) panicked
  ...
  Pause(Error signalling vCPUs: Timeout when waiting for signal
        to be acknowledged)

The vCPU blocks on the activation barrier and never reaches the normal
pause checkpoint. Later, migration enters downtime and stops the vCPU
throttle thread. In the failing case, that thread is still inside a
CpuManager::pause() call, which waits for every vCPU to acknowledge
the signal. The blocked vCPU never does, so the pause times out.

The VMM already receives ActivateVirtioDevices events during
migration, but it only drains pending activations when self.vm is in
MaybeVmOwnership::Vmm. Once vm_send_migration() moves the Vm into the
migration worker, self.vm becomes MaybeVmOwnership::Migration and the
event handler no longer has a path to call activate_virtio_devices().

Fix this by storing the DeviceManager inside
MaybeVmOwnership::Migration. This keeps just enough state on the VMM
thread to drain pending virtio activations while the migration worker
owns the Vm. The barrier logic stays unchanged. The VMM now releases
the same activation barrier during migration that it already released
before migration started.

This keeps the guest from getting stuck in the activation wait and
lets the later pause succeed.

See reproducer in libvirt-tests

Steps to un-draft

• verify if there is anything left for https://github.com/cobaltcore-dev/cobaltcore/issues/411
• Wait for Pipeline to go green
• drop xxx commit

[tpressure]

Very nice debugging!

[tpressure]

@Coffeeri, can you please add a test in libvirt-tests that does a couple of migrations directly after the vm boots?

[phip1611]

LGTM