reduce max size of additional disk by server disk min

app/UI/VmCreate/Sections/AdditionalDiskSection.php

@@ -31,7 +31,7 @@ class AdditionalDiskSection extends BaseSection implements ClientArea
     {
         //size
         $this->resourceManager = new ResourceManager();
-        $field = new Range('additionalDiskSize', $this->configuration()->serverDiskSize->min, $this->resourceManager->disk()->free());
+        $field = new Range('additionalDiskSize', $this->configuration()->serverDiskSize->min, $this->resourceManager->disk()->free() - $this->configuration()->serverDiskSize->min);
         $field->addValidator(new DiskSizeValidator('additionalDiskSize',true));
         $field->setDefaultValue($this->configuration()->serverDiskSize->min);
         $field->setDescription('description');