bcvk-qemu: Add MachineType for architecture-appropriate defaults

Per previous commit we're still carrying direct qemu code, and we
really do want to use the modern default machine types. This came
up as part of deduplicating our qemu logic with that generated
for custom bootc testing.

This mirrors the machine types used by bcvk's libvirt integration
(see crates/kit/src/arch.rs) for consistency between direct QEMU
and libvirt code paths.

Assisted-by: OpenCode (claude-opus-4-5@20251101)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/bcvk-qemu/src/lib.rs

@@ -53,8 +53,8 @@ pub use credentials::{
 };
 
 pub use qemu::{
-    BootMode, DiskFormat, DisplayMode, NetworkMode, QemuConfig, ResourceLimits, RunningQemu,
-    VirtioBlkDevice, VirtioSerialOut, VirtiofsMount, VHOST_VSOCK,
+    BootMode, DiskFormat, DisplayMode, MachineType, NetworkMode, QemuConfig, ResourceLimits,
+    RunningQemu, VirtioBlkDevice, VirtioSerialOut, VirtiofsMount, VHOST_VSOCK,
 };
 
 pub use virtiofsd::{spawn_virtiofsd_async, validate_virtiofsd_config, VirtiofsConfig};

crates/bcvk-qemu/src/qemu.rs

@@ -118,6 +118,38 @@ pub struct ResourceLimits {
     pub nice_level: Option<i8>,
 }
 
+/// QEMU machine type selection.
+#[derive(Debug, Clone, Default)]
+pub enum MachineType {
+    /// Auto-detect based on host architecture (recommended).
+    #[default]
+    Auto,
+    /// Use a specific machine type string (e.g., "q35", "pc-q35-9.2").
+    Explicit(String),
+}
+
+impl MachineType {
+    /// Resolve to a QEMU `-machine` argument for the current host.
+    ///
+    /// Returns `None` for unknown architectures, letting QEMU use its default.
+    pub fn resolve(&self) -> Option<&str> {
+        // xref: https://github.com/coreos/coreos-assembler/blob/main/mantle/platform/qemu.go
+        match self {
+            Self::Auto => match std::env::consts::ARCH {
+                "x86_64" => Some("q35"),
+                // gic-version=max selects the best available GIC for the host
+                "aarch64" => Some("virt,gic-version=max"),
+                "s390x" => Some("s390-ccw-virtio"),
+                // kvm-type=HV ensures bare metal KVM, not user mode
+                // ic-mode=xics for interrupt controller
+                "powerpc64" => Some("pseries,kvm-type=HV,ic-mode=xics"),
+                _ => None,
+            },
+            Self::Explicit(name) => Some(name.as_str()),
+        }
+    }
+}
+
 /// VM boot configuration.
 #[derive(Debug)]
 pub enum BootMode {
@@ -157,6 +189,8 @@ pub struct QemuConfig {
     pub memory_mb: u32,
     /// Number of vCPUs (1-256).
     pub vcpus: u32,
+    /// Machine type (default: auto-detect based on host architecture).
+    pub machine_type: MachineType,
     boot_mode: Option<BootMode>,
     /// Main VirtioFS configuration for root filesystem (handled separately from additional mounts).
     pub main_virtiofs_config: Option<VirtiofsConfig>,
@@ -488,6 +522,11 @@ fn spawn(
         });
     }
 
+    // Set machine type (auto-detected or explicit)
+    if let Some(machine) = config.machine_type.resolve() {
+        cmd.args(["-machine", machine]);
+    }
+
     cmd.args([
         "-m",
         &memory_arg,