QEMU microvm support

[JayJamieson]

Is there any chance to add support for QEMU microvm machine type?

Microvm machine type would permit faster boots and somewhat lower overhead.

If allowed, I would be happy to open a draft PR for review and testing.

[rgarcia]

Implemented and merged in #376. Hypeman now supports hypervisor: qemu-microvm as a first-class Linux amd64 backend, including networking, vsock, ballooning, stop/start, snapshots, standby restore, and warm forks.

The backend enforces microVM’s constraints: no PCI/vGPU, no memory hotplug, and at most eight virtio-mmio devices.

Thanks for opening this and offering to contribute!