feat(firecracker): persistent registry volume for the infra VM (Stage B, 6/n)

The infra VM's rootfs is ephemeral (rebuilt each boot), so the bottle
registry DB needs durable storage across restarts. Give the infra VM a
firecracker analogue of a docker volume: a host-side ext4 file attached as
a second virtio-block device (guest /dev/vdb), mounted at the control
plane's DB dir (/var/lib/bot-bottle, where host_db_path lives at
db/bot-bottle.db).

- firecracker_vm.boot/_config take an optional `data_drive` (a non-root,
  RW second drive).
- infra_vm creates the volume on first use (`mke2fs` an empty ext4 at
  <fc-cache>/infra/registry.ext4) and mounts /dev/vdb in the PID-1 init
  before the control plane starts. It's a plain ext4 file, so
  `sudo mount -o loop <path>` (VM stopped) inspects bot-bottle.db directly.

Verified on a KVM host: register a bottle, restart the infra VM (fresh
rootfs, same volume) — /dev/vdb re-mounts and the registry `db/` dir + a
marker file survive the restart.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/backend/firecracker/firecracker_vm.py

@@ -78,20 +78,32 @@ def _config(
     vcpus: int,
     mem_mib: int,
     guest_mac: str,
+    data_drive: Path | None = None,
 ) -> dict[str, object]:
+    drives: list[dict[str, object]] = [
+        {
+            "drive_id": "rootfs",
+            "path_on_host": str(rootfs),
+            "is_root_device": True,
+            "is_read_only": False,
+        }
+    ]
+    # A second virtio-block device (guest /dev/vdb) — the infra VM's
+    # persistent registry "volume", a host-side ext4 file that outlives the
+    # ephemeral rootfs across VM restarts.
+    if data_drive is not None:
+        drives.append({
+            "drive_id": "data",
+            "path_on_host": str(data_drive),
+            "is_root_device": False,
+            "is_read_only": False,
+        })
     return {
         "boot-source": {
             "kernel_image_path": str(util.kernel_path()),
             "boot_args": _boot_args(guest_ip, host_ip, pubkey),
         },
-        "drives": [
-            {
-                "drive_id": "rootfs",
-                "path_on_host": str(rootfs),
-                "is_root_device": True,
-                "is_read_only": False,
-            }
-        ],
+        "drives": drives,
         "network-interfaces": [
             {
                 "iface_id": "eth0",
@@ -119,6 +131,7 @@ def boot(
     mem_mib: int = 2048,
     guest_mac: str = "06:00:AC:10:00:02",
     detached: bool = False,
+    data_drive: Path | None = None,
 ) -> VmHandle:
     """Write the config and launch the VMM. Returns once the process is
     spawned; callers wait for SSH readiness separately.
@@ -134,6 +147,7 @@ def boot(
         _config(
             rootfs=rootfs, tap=tap, guest_ip=guest_ip, host_ip=host_ip,
             pubkey=pubkey, vcpus=vcpus, mem_mib=mem_mib, guest_mac=guest_mac,
+            data_drive=data_drive,
         ),
         indent=2,
     ))