Fix ubuntu hang

This seems to be the problem with the box. I tried to solve this problem locally.

1. use nc to connect to the serial port (See Debug part of the readme)

   #  get the id of this virtual machine  
   cat .vagrant/machines/default/qemu/id
   k-2sGaa94eE%
   
   # get the path to qemu_socket_serial
   ls ~/.vagrant.d/tmp/vagrant-qemu/k-2sGaa94eE/qemu_socket_serial
   
   # connect to it
   nc -U ~/.vagrant.d/tmp/vagrant-qemu/k-2sGaa94eE/qemu_socket_serial
   
   # press Enter, it may show
   ubuntu login:
   

2. you may see different outputs

   • boot message, wait to login
   • login prompt, login with username/password: vagrant/vagrant
   • others, please report to this issue

3. after login to ubuntu, enable the network

   # check network, you will see that eth0 has no ip address
   ip a
   
   # update netplan config
   sudo su -
   sed -i -e 's/enp[0-9]s[0-9]/eth0/' /etc/netplan/00-installer-config.yaml
   
   # power off
   poweroff
   

4. wait a few minutes, until ps -ef|grep qemu shows no running qemu process

5. restart machine: vagrant up

Notes, you also need add config.vm.synced_folder ".", "/vagrant", type: "smb", smb_host: "10.0.2.2" to the Vagrantfile due to a know issue.