qemuBuildHostNetStr: Explicitly enumerate net types
    
    We tend to prevent using 'default' in switches. And it is for a
    good reason - control may end up in paths we wouldn't want for
    new values. In this specific case, if qemuBuildHostNetStr is
    called over VIR_DOMAIN_NET_TYPE_VHOSTUSER it would produce
    meaningless output. Fortunately, there no such call yet.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>