Split the QEMU configs by host system and architecture

Replace qemu/<guest>-<arch>.ini with qemu/<guest>-<os>-<arch>.ini, where os is
linux or darwin, the name platform.system() reports. The accelerators move out
of the shared <guest>.ini into these files (KVM on Linux, HVF on macOS, TCG as
the fallback). The launchers pick the file from the host system and
architecture, and reject amd64 guests on macOS hosts, so no darwin-amd64 files
exist.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

run-ubuntu2604.py

@@ -70,7 +70,10 @@ host_arch = normalize_arch(env("VM_IMAGE_ARCH") or native_arch)
 qemu_binary = "qemu-system-aarch64" if host_arch == "arm64" else "qemu-system-x86_64"
 qemu_config_template = Path(
     env("UBUNTU_VM_QEMU_CONFIG") or script_dir / "qemu" / "ubuntu2604.ini")
-qemu_arch_config = script_dir / "qemu" / f"ubuntu2604-{host_arch}.ini"
+host_os = platform.system().lower()
+if host_os == "darwin" and host_arch == "amd64":
+    die("amd64 guests are not supported on macOS hosts; use an arm64 image")
+qemu_platform_config = script_dir / "qemu" / f"ubuntu2604-{host_os}-{host_arch}.ini"
 
 
 def pick_firmware(override, *candidates):
@@ -126,7 +129,7 @@ created in the state directory (default: .vm/ubuntu2604/), so the downloaded
 base image is never modified. The VM hardware (memory, CPUs, SSH port forward,
 ...) is defined in qemu/ubuntu2604.ini, which is rendered to qemu.ini in the
 state directory and loaded with QEMU's -readconfig, followed by
-qemu/ubuntu2604-<arch>.ini. Later QEMU arguments override them, e.g.
+qemu/ubuntu2604-<os>-<arch>.ini (os is linux or darwin, arch is amd64 or arm64). Later QEMU arguments override them, e.g.
 `-- -m 4G -smp 4`.
 
 Options:
@@ -253,7 +256,7 @@ for key, value in config_values.items():
     # QEMU's config file parser has no escape syntax for quoted values.
     if any(char in str(value) for char in '"\n'):
         die(f"{key} must not contain double quotes or newlines: {value}")
-for config_file in (qemu_config_template, qemu_arch_config):
+for config_file in (qemu_config_template, qemu_platform_config):
     if not config_file.is_file():
         die(f"QEMU config file was not found: {config_file}")
 try:
@@ -265,7 +268,7 @@ qemu_config.write_text(qemu_config_text)
 
 qemu_args = [
     "-readconfig", str(qemu_config),
-    "-readconfig", str(qemu_arch_config),
+    "-readconfig", str(qemu_platform_config),
     # "max" is the host CPU under KVM/HVF and the fullest model under TCG.
     "-cpu", "max",
     # The config file's [netdev] does not stop QEMU from adding its default
@@ -275,7 +278,7 @@ qemu_args = [
 ]
 
 print(f"Starting Ubuntu 26.04 ({host_arch}) from {qemu_config_template} "
-      f"and {qemu_arch_config}.", flush=True)
+      f"and {qemu_platform_config}.", flush=True)
 # The SSH port forward lives in the config file, e.g. hostfwd = "tcp::2225-:22".
 ssh_forward = re.search(r'hostfwd\s*=\s*"tcp:[^:"]*:(\d+)-[^:"]*:22"', qemu_config_text)
 if ssh_forward: