machines: fix wrongly backported state variable name

On master it is format but here it should be diskFileFormat.

Author: KKoukiou

pkg/machines/components/diskAdd.jsx

@@ -439,7 +439,7 @@ class AddDiskModalBody extends React.Component {
             // Reset the format only when the Format selection dropdown changes entries - otherwise just keep the old selection
             // All pool types apart from 'disk' have either 'raw' or 'qcow2' format
             if ((currentPool.type == 'disk' && prevPool.type != 'disk') || (currentPool.type != 'disk' && prevPool.type == 'disk'))
-                this.onValueChanged('format', this.getDefaultVolumeFormat(value));
+                this.onValueChanged('diskFileFormat', this.getDefaultVolumeFormat(value));
 
             if (this.state.mode === USE_EXISTING) { // user changed pool
                 this.onValueChanged('existingVolumeName', this.getDefaultVolumeName(value));