qemuBuildHostNetStr: use type_sep earlier
    
    When hotplugging networks with ancient QEMUs not supporting
    QEMU_CAPS_NETDEV, we use space instead of a comma as the separator
    between the network type and other options.
    
    Except for "user", all the network types pass other options
    and use up the first separator by the time we get to the section
    that adds the alias (or vlan for QEMUs without CAPS_NETDEV).
    
    Since the alias/vlan is mandatory, convert all preceding code to add
    the separator at the end, removing the need to rewrite type_sep for
    all types but NET_TYPE_USER.