feat: add host-driven balloon controls

[enricoschaaf]

Summary

Adds host-driven virtio-balloon control to libkrun and exposes it through both the native Rust API and the C API.

This lets callers configure a VM memory ceiling, boot the guest with a lower guest-visible memory target, and adjust that target at runtime through a host-side Unix socket.

Technical implementation

• Extends the virtio-balloon device with a cloneable BalloonControl handle backed by an atomic target page count and an eventfd registered in the VMM event loop.
• Changes Balloon::new() to accept initial and maximum balloon targets in virtio-balloon page units. The initial target seeds num_pages, so the guest sees the desired target during driver initialization.
• Handles the inflate and deflate virtqueues instead of logging them as unsupported:
  • inflate descriptors are parsed as 4 KiB PFNs and each page is released back to the host with madvise (MADV_FREE_REUSABLE on macOS, MADV_DONTNEED elsewhere);
  • deflate descriptors are acknowledged so the guest can reuse those pages;
  • free-page reporting continues to reclaim descriptor ranges with the same platform-specific advice.
• Accepts guest writes to the actual config field and signals config-change interrupts when the host target changes, which is the virtio-balloon mechanism the guest driver uses to observe a new num_pages target.
• Adds VmResources::balloon as a small BalloonResize struct containing initial and maximum reclaimed page counts. build_microvm() passes this into balloon device attachment and stores the BalloonControl on Vmm when runtime control is enabled.
• Adds VmBuilder::balloon(...) for the Rust API. It validates initial_mib <= memory_mib, min_mib <= memory_mib, and min_mib <= initial_mib, then converts guest-visible MiB values into reclaimed 4 KiB pages relative to the configured memory ceiling.
• Adds a per-VM Unix stream socket listener. Each accepted connection reads a decimal MiB target, converts it to reclaimed pages relative to the ceiling, updates the BalloonControl, and writes an OK <pages> or ERR ... response.
• Adds the C API entry point krun_set_balloon(ctx_id, initial_mib, min_mib, control_socket_path) with the same guest-visible MiB semantics and a TEE stub returning EOPNOTSUPP.
• Propagates blk and net feature flags through src/libkrun/Cargo.toml, matching the existing Rust crate feature propagation, so the C library crate builds with the expected optional device stacks.
• Bumps the msb_krun* crate versions to 0.1.15 so downstream code can depend on this API distinctly from the currently published crate.

Validation

• cargo check -p msb_krun --features 'blk net'
• cargo test -p msb_krun_devices balloon --lib
• cargo check -p libkrun --features 'blk net'
• cargo check -p msb_krun_vmm

I did not run a VM smoke test for this PR; this repo does not include a lightweight local smoke harness for exercising a full balloon resize path without a booted guest.