Updated things related to the general hw config in the virtualbox builders to correspond to @azr's suggestions.

builder/virtualbox/common/hw_config.go

@@ -23,7 +23,6 @@ func (c *HWConfig) Prepare(ctx *interpolate.Context) []error {
 	// Hardware and cpu options
 	if c.CpuCount < 0 {
 		errs = append(errs, fmt.Errorf("An invalid number of cpus was specified (cpus < 0): %d", c.CpuCount))
-		c.CpuCount = 0
 	}
 	if c.CpuCount == 0 {
 		c.CpuCount = 1
@@ -31,7 +30,6 @@ func (c *HWConfig) Prepare(ctx *interpolate.Context) []error {
 
 	if c.MemorySize < 0 {
 		errs = append(errs, fmt.Errorf("An invalid memory size was specified (memory < 0): %d", c.MemorySize))
-		c.MemorySize = 0
 	}
 	if c.MemorySize == 0 {
 		c.MemorySize = 512
@@ -42,5 +40,5 @@ func (c *HWConfig) Prepare(ctx *interpolate.Context) []error {
 		c.Sound = "none"
 	}
 
-	return nil
+	return errs
 }