This commit creates a custom `read_dhcp_servers` method in the
VirtualBox 6.1 driver to handle changes made in the ouput of
`VBoxManage list dhcpservers`.
Tests for VirtualBox 6.1+ can no longer use the shared examples for the
VirtualBox 4.x driver, because the `read_dhcp_servers` change is not
backwards compatible.
This commit also creates the boilerplate for a VirtualBox 6.x shared
example in case we want to put tests there in the future.
