By Lizbeth Diaz MEXICO CITY (Reuters) – Ana Villalobos remembers what she went through to get from Venezuela to the United States: going hungry, sleeping in the open and crossing rivers and forests with her young son. She cries upon reflecting on the journey, knowing it bore no fruit. Now, Villalobos is asking for a humanitarian flight to return to her homeland. Tens of thousands of Venezuelan migrants are now stranded in Mexico or its southern neighbors after a new plan was announced by the United States that, in practice, has closed the door to Venezuelans fleeing on foot. Dozens of Venezuel…