builder/qemu: add simple kvm/tcg autodetection

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>

builder/qemu/builder.go

@@ -154,8 +154,16 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		if runtime.GOOS == "windows" {
 			b.config.Accelerator = "tcg"
 		} else {
-			b.config.Accelerator = "kvm"
+			if fp, err := os.Open("/dev/kvm"); err != nil {
+				b.config.Accelerator = "tcg"
+			} else {
+				fp.Close()
+				b.config.Accelerator = "kvm"
+			}
 		}
+		log.Printf("use detected accelerator: %s", b.config.Accelerator)
+	} else {
+		log.Printf("use specified accelerator: %s", b.config.Accelerator)
 	}
 
 	if b.config.MachineType == "" {