kvm2: Require Ubuntu >=18.04
kvm2: Require Ubuntu >=18.04

Mention that the instruction in this section doesn't work for old versions
of Ubuntu as it is.

Because the provided driver binary requires a later version of libvirt.

    % ./docker-machine-driver-kvm2
    ./docker-machine-driver-kvm2: /usr/lib/x86_64-linux-gnu/libvirt-lxc.so.0: version `LIBVIRT_LXC_2.0.0' not found (required by ./docker-machine-driver-kvm2)
    ./docker-machine-driver-kvm2: /usr/lib/x86_64-linux-gnu/libvirt.so.0: version `LIBVIRT_2.2.0' not found (required by ./docker-machine-driver-kvm2)
    ./docker-machine-driver-kvm2: /usr/lib/x86_64-linux-gnu/libvirt.so.0: version `LIBVIRT_3.0.0' not found (required by ./docker-machine-driver-kvm2)
    ./docker-machine-driver-kvm2: /usr/lib/x86_64-linux-gnu/libvirt.so.0: version `LIBVIRT_1.3.3' not found (required by ./docker-machine-driver-kvm2)
    ./docker-machine-driver-kvm2: /usr/lib/x86_64-linux-gnu/libvirt.so.0: version `LIBVIRT_2.0.0' not found (required by ./docker-machine-driver-kvm2)
    %

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