feat(virtio): add generic vhost-user frontend device

[meAmitPatil]

Implement a generic vhost-user frontend device that is agnostic to the specific virtio device type being emulated. Unlike per-device-type frontends, this device delegates config space ownership entirely to the backend via the mandatory CONFIG protocol feature, allowing any virtio device type (e.g. virtio-fs, virtio-scsi) to be used without a
dedicated Firecracker frontend.

A VirtioDeviceType::VhostUserGeneric sentinel (0xFF) serves as the host-side MMIO map key, while a new mmio_device_type_id() trait method returns the real virtio type ID to the guest MMIO register. Queue count
is dynamic (Vec) since different device types need different configurations. Snapshotting is stubbed out, consistent with the existing vhost-user block device.

The device is configured via PUT /vhost-user-devices/{id} and tested end-to-end with virtiofsd — guest kernel recognises the device with the correct virtio type ID.

Closes firecracker-microvm#5687

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• This functionality cannot be added in rust-vmm.