feat: add host-backed entropy device support and update kernel configuration

uvm/firecracker/api.py

@@ -35,12 +35,15 @@ class FirecrackerClient:
         boot = config["boot-source"]
         drives = config["drives"]
         interfaces = config["network-interfaces"]
+        entropy = config.get("entropy")
         if not isinstance(machine, Mapping) or not isinstance(boot, Mapping):
             raise FirecrackerError("invalid Firecracker configuration")
         if not isinstance(drives, list) or not drives:
             raise FirecrackerError("Firecracker configuration has no root drive")
         if not isinstance(interfaces, list) or not interfaces:
             raise FirecrackerError("Firecracker configuration has no network interface")
+        if not isinstance(entropy, Mapping):
+            raise FirecrackerError("Firecracker configuration has no valid entropy device")
 
         self._request(
             "PUT",
@@ -54,6 +57,7 @@ class FirecrackerClient:
         self._request("PUT", "/boot-source", boot)
         self._request("PUT", "/drives/rootfs", drives[0])
         self._request("PUT", "/network-interfaces/eth0", interfaces[0])
+        self._request("PUT", "/entropy", entropy)
         self._request("PUT", "/actions", {"action_type": "InstanceStart"})
 
     def _request(self, method: str, path: str, body: object) -> None:

uvm/firecracker/config.py

@@ -70,6 +70,8 @@ def build_config(
                 "host_dev_name": vm.tap,
             }
         ],
+        # Supply host-backed randomness before guest services such as sshd start.
+        "entropy": {},
     }