Implement default hypervisor options on settings (#573)

Problem: The unique way to set the default hypervisor to use is hardcoded on the `models.py` class, so we can't select another one by default.

Solution: Implement `INSTANCE_DEFAULT_HYPERVISOR` field on the orchestrator configuration variables.

src/aleph/vm/conf.py

@@ -11,6 +11,7 @@
 from subprocess import CalledProcessError, check_output
 from typing import Any, Literal, NewType, Optional, Union
 
+from aleph_message.models.execution.environment import HypervisorType
 from pydantic import BaseSettings, Field, HttpUrl
 from pydantic.env_settings import DotenvType, env_file_sentinel
 from pydantic.typing import StrPath
@@ -255,6 +256,10 @@ class Settings(BaseSettings):
     ALLOCATION_TOKEN_HASH = "151ba92f2eb90bce67e912af2f7a5c17d8654b3d29895b042107ea312a7eebda"
 
     ENABLE_QEMU_SUPPORT: bool = Field(default=False)
+    INSTANCE_DEFAULT_HYPERVISOR: Optional[HypervisorType] = Field(
+        default=HypervisorType.firecracker,  # User Firecracker
+        description="Default hypervisor to use on running instances, can be Firecracker or QEmu",
+    )
 
     # Tests on programs
 
@@ -391,6 +396,10 @@ def setup(self):
                 network_interface=self.NETWORK_INTERFACE,
             )
 
+        if not settings.ENABLE_QEMU_SUPPORT:
+            # If QEmu is not supported, ignore the setting and use Firecracker by default
+            settings.INSTANCE_DEFAULT_HYPERVISOR = HypervisorType.firecracker
+
     def display(self) -> str:
         attributes: dict[str, Any] = {}
 

src/aleph/vm/models.py

@@ -105,8 +105,11 @@ def is_instance(self) -> bool:
 
     @property
     def hypervisor(self) -> HypervisorType:
-        # default to firecracker for retro compat
-        return self.message.environment.hypervisor or HypervisorType.firecracker
+        if self.is_program:
+            return HypervisorType.firecracker
+
+        # Hypervisor setting is only used for instances
+        return self.message.environment.hypervisor or settings.INSTANCE_DEFAULT_HYPERVISOR
 
     @property
     def becomes_ready(self) -> Callable[[], Coroutine]: