Removing the "no buildable source files" error on non-linux machines

cmd/drivers/kvm/main-nolinux.go

+// +build !linux
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	fmt.Println(
+		"this driver was built on a non-linux machine, so it is " +
+			"unavailable. Please re-build minikube on a linux machine to enable " +
+			"it.",
+	)
+	os.Exit(1)
+}