I booted up CentOS 7 x86_64 Minimal ISO in VirtualBox but it came without pre-enabled networking. According to CentOS 7 FAQ, you can easily enable networking at boot using the following command.
sed -i -e 's@^ONBOOT="no@ONBOOT="yes@' ifcfg-enp0s3
Remember to make sure to edit the correct interface. In my case it was ifcfg-enp0s3 which is based on the new naming scheme of network interfaces.