DriveSetup: automatically select single disk

When there is only one disk, automatically select it.
Otherwise, do not select anything to make sure the user does not
accidentally do things to the wrong disk.

Change-Id: I834415bda6bc43692697bdfa9030f35ffc94048f
Reviewed-on: https://review.haiku-os.org/c/1397
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/apps/drivesetup/DiskView.cpp

@@ -439,6 +439,11 @@ void
 DiskView::SetDiskCount(int32 count)
 {
 	fDiskCount = count;
+	if (count == 1) {
+		BMessage message(MSG_SELECTED_PARTITION_ID);
+		message.AddInt32("partition_id", 0);
+		Window()->PostMessage(&message);
+	}
 }