do validation in vmx config stage

builder/virtualbox/ovf/config.go

@@ -2,6 +2,7 @@ package ovf
 
 import (
 	"fmt"
+	"os"
 	"strings"
 
 	vboxcommon "github.com/hashicorp/packer/builder/virtualbox/common"
@@ -101,6 +102,13 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		if err != nil {
 			errs = packer.MultiErrorAppend(errs, fmt.Errorf("source_path is invalid: %s", err))
 		}
+		// file must exist now.
+		if (len(c.SourcePath) > 7) && (c.SourcePath[:7] == "file://") {
+			if _, err := os.Stat(c.SourcePath[7:]); err != nil {
+				errs = packer.MultiErrorAppend(errs,
+					fmt.Errorf("source file needs to exist at time of config validation: %s", err))
+			}
+		}
 	}
 
 	validMode := false

builder/virtualbox/ovf/config_test.go

@@ -65,14 +65,14 @@ func TestNewConfig_sourcePath(t *testing.T) {
 		t.Fatalf("should error with empty `source_path`")
 	}
 
-	// Okay, because it gets caught during download
+	// Want this to fail on validation
 	c = testConfig(t)
 	c["source_path"] = "/i/dont/exist"
 	_, warns, err = NewConfig(c)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
-	if err != nil {
+	if err == nil {
 		t.Fatalf("bad: %s", err)
 	}
 
@@ -84,7 +84,7 @@ func TestNewConfig_sourcePath(t *testing.T) {
 		t.Fatalf("bad: %#v", warns)
 	}
 	if err == nil {
-		t.Fatal("should error")
+		t.Fatal("Nonexistent source file should be caught in validation")
 	}
 
 	// Good