try to use slider for additional disk in createVM

app/UI/VmCreate/Sections/AdditionalDiskSection.php

@@ -29,6 +29,7 @@ class AdditionalDiskSection extends BaseSection implements ClientArea
     public function initFields()
     {
         //size
+        $this->resourceManager = new ResourceManager();
         $field = new Range('additionalDiskSize', $this->configuration()->serverDiskSize->min, $this->resourceManager->disk()->free());
         $field->addValidator(new DiskSizeValidator('additionalDiskSize',true));
         $field->setDefaultValue($this->configuration()->serverDiskSize->min);