fix: add device polling for config disk to prevent race condition

The device polling optimization waited only for /dev/vda and /dev/vdb,
but /dev/vdc (config disk) was mounted without polling. This could cause
mount failures if vdc appeared slightly after vdb.

Now readConfig() polls for /dev/vdc before attempting to mount it,
consistent with how we handle the other block devices.

Author: Hiro Tamada

lib/system/init/config.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"time"
 
 	"github.com/kernel/hypeman/lib/vmconfig"
 )
@@ -19,6 +20,11 @@ func readConfig(log *Logger) (*vmconfig.Config, error) {
 		return nil, fmt.Errorf("mkdir config mount: %w", err)
 	}
 
+	// Wait for config disk to be ready (polls every 10ms, 2s timeout)
+	if err := waitForDevice("/dev/vdc", 2*time.Second); err != nil {
+		return nil, fmt.Errorf("wait for config device: %w", err)
+	}
+
 	// Mount config disk (/dev/vdc) read-only
 	cmd := exec.Command("/bin/mount", "-o", "ro", "/dev/vdc", configMount)
 	if output, err := cmd.CombinedOutput(); err != nil {