First enable/add NatNetwork and Host only Network in VirtualBox Interface
File > Preferences > Network

If Host-only adapter gives error on Windows go to
Control Pannel > Network and Sharing Center > Change Adapter Settings > Virtual Host-only Network > Preferences
Check VirtualBox __________

In the Network section of the new instance two Adapters should be visible
1. NAT
2. Host-only Adapter

Set ISO image and start intsallation
* On setup enable both network adapters
* Set hostname – server1.example.com/ server2.example.com
* Set Timezone
* Create root password
* Create user

After installation
make sure both connections are on at boot

sed -i s/ONBOOT=no/ONBOOT=yes /etc/sysconfig/network-scripts/ifcfg-enp0s3
sed -i s/ONBOOT=no/ONBOOT=yes /etc/sysconfig/network-scripts/ifcfg-enp0s8

Update software and install

yum update
yum install -y redhat-lsb-core net-tools epel-release kernel-headers kernel-devel
yum groupinstall -y "Develpoment Tools"
yum groupinstall -y "X Window System" "MATE Desktop"

Set graphical.target as default

// create symlink
// from /etc/systemd/system/default.target to graphical.target
systemctl set-default graphical.target
systemctl isolate graphical-target

Reboot to make sure you are using the latest kernel.

Install GuestAdditions.iso for Virtualbox

Go to Devices > Insert Guest Additions CD image > Run

su -
mount
/run/media/maja/VBOXADDITIONS_5.1.18_114002/VBoxLinuxAdditions.run

On Ubuntu you can download ISO with:

sudo apt-get install virtualbox-guest-additions-iso