admin: Introduce virAdmConnectGetLibVersion
    
    Introduce a new API to get libvirt version. It is worth noting, that
    libvirt-admin and libvirt share the same version number. Unfortunately,
    our existing API isn't generic enough to be used with virAdmConnectPtr
    as well. Also this patch wires up this API to the virt-admin client
    as a generic cmdVersion command.