[Request]: Cloud Hypervisor backend: virtio-balloon config and vm.resize #978

Feature or enhancement request details

The Cloud Hypervisor backend has no way to attach or drive a virtio-balloon. Cloud Hypervisor supports one (balloon in vm.create's VmConfig, and desired_balloon in vm.resize), but CloudHypervisor.VmConfig / DeviceConfigs have no balloon type and the client has no vm.resize endpoint, so even a CHInstanceExtension.configureCH cannot add one. The instance's REST client is internal, so a caller also can't resize it after boot.

#882 proposed balloon support across both backends together with a reclaim policy; it and its PRs (#893, #894) were closed without discussion. This request is only the mechanism on the Cloud Hypervisor side, which is additive and policy-free:

• CloudHypervisor.BalloonConfig (size, deflate_on_oom, free_page_reporting) and VmConfig.balloon: BalloonConfig?
• CloudHypervisor.VmResize and Client.vmResize(_:) → PUT /api/v1/vm.resize
• CHVirtualMachineInstance.setBalloonSize(_:) and memoryActualSize() (from vm.info's memory_actual_size)

Usage:

struct Balloon: CHInstanceExtension {
    func configureCH(_ config: inout CloudHypervisor.VmConfig) throws {
        config.balloon = .init(size: 0, deflateOnOom: false)
    }
}
// ... after start:
try await instance.setBalloonSize(512 * 1024 * 1024)
let actual = try await instance.memoryActualSize()

Checked on Cloud Hypervisor v52.0, x86_64 KVM, 1 GiB guest: a 512 MiB balloon takes memory_actual_size from 1152 to 640 MiB and the guest's MemTotal from 1122276 to 597988 KiB (exactly 512 MiB); setting it back to 0 restores both.

Implementation with unit tests (made against 0.47.0; cherry-picks cleanly onto main): kunalkushwaha@bc78926 — happy to open a PR against main if this is wanted.

Note the semantics differ from VZ's VZVirtioTraditionalMemoryBalloonDevice.targetVirtualMachineMemorySize (memory the guest keeps) — Cloud Hypervisor's balloon size is memory taken from the guest. A cross-backend API would need to convert; this request doesn't propose one.

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