[codex] add linux amd64 virtio balloon support

[Vbitz]

Summary

Adds Linux/amd64 KVM support for the virtio balloon work from the Darwin branch.

• wires virtio.Balloon into KVM managed sessions, one-shot exec, MMIO dispatch, and startup snapshot state
• passes requested balloon targets through Linux managed attachments and runtime requests
• reclaims inflated guest pages with madvise(MADV_DONTNEED) while preserving the existing guest memory mapping
• preserves existing amd64 MMIO addresses and adds the balloon at a new slot

Validation

• go test ./internal/hv/kvm ./internal/virtio

Full go test ./... still reaches existing Linux runtime tests that require an embedded amd64 guest-init payload (build ccvm with -tags embed_guestinit).

[Vbitz]

Follow-up pushed: 57cea75 loads CONFIG_VIRTIO_BALLOON / virtio_balloon.ko.gz for Linux/amd64 and broadens the existing virtio balloon runtime boot test to cover Linux/amd64.

Additional validation now passing locally:

• go test ./internal/vm -run TestRuntimeBootsLinuxWithVirtioBalloon -count=1
• go test ./internal/hv/kvm ./internal/virtio ./internal/vm -run 'TestRuntimeBootsLinuxWithVirtioBalloon|TestLinuxManagedMachineSpec'

The first test boots a real Linux/amd64 guest and verifies the virtio balloon device is bound to the virtio_balloon driver.

[Vbitz]

Rebased onto cc/main now that Darwin balloon support has merged. The PR now contains only the Linux amd64 balloon commits (eda86e5, ee12a10).

Validation run locally:

• go test ./cmd/ccvm ./cmd/cc ./internal/guestinit ./internal/linux/boot/amd64 ./internal/linux/initramfs -count=1
• go test ./internal/hv/kvm ./internal/vm ./internal/vm/host/kvm -count=1 reached the known local internal/vm startup-snapshot FPU guest panic, while internal/hv/kvm and internal/vm/host/kvm passed.