If found that this issue is related to resolv.conf of the host machine. It points to a dnsmasq of localhost instead of the DNS servers provided by the network connection(s). To disable this dnsmasq "feature", edit /etc/NetworkManager/NetworkManager.conf and add a hash sign # right in front of dns=dnsmasq. Then restart the NetworkManager with service network-manager restart. Finally, stop all virtual machines and boot them.