Add README

Author: sjmiller609

lib/hypervisor/README.md

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+# Hypervisor Abstraction
+
+Provides a common interface for VM management across different hypervisors.
+
+## Purpose
+
+Hypeman originally supported only Cloud Hypervisor. This abstraction layer allows supporting multiple hypervisors (e.g., QEMU) through a unified interface, enabling:
+
+- **Hypervisor choice per instance** - Different instances can use different hypervisors
+- **Feature parity where possible** - Common operations work the same way
+- **Graceful degradation** - Features unsupported by a hypervisor can be detected and handled
+
+## How It Works
+
+The abstraction defines two key interfaces:
+
+1. **Hypervisor** - VM lifecycle operations (create, boot, pause, resume, snapshot, restore, shutdown)
+2. **ProcessManager** - Hypervisor process lifecycle (start binary, get binary path)
+
+Each hypervisor implementation translates the generic configuration and operations to its native format. For example, Cloud Hypervisor uses an HTTP API over a Unix socket, while QEMU would use QMP.
+
+Before using optional features, callers check capabilities:
+
+```go
+if hv.Capabilities().SupportsSnapshot {
+    hv.Snapshot(ctx, path)
+}
+```
+
+## Hypervisor Switching
+
+Instances store their hypervisor type in metadata. An instance can switch hypervisors only when stopped (no running VM, no snapshot), since:
+
+- Disk images are hypervisor-agnostic
+- Snapshots are hypervisor-specific and cannot be restored by a different hypervisor