lxc: always use virDomainNetGetActualBridgeName to get interface's bridge
    
    lxcProcessSetupInterfaces() used to have a special case for
    actualType='network' (a network with forward mode of route, nat, or
    isolated) to call the libvirt public API to retrieve the bridge being
    used by a network. That is no longer necessary - since all network
    types that use a bridge and tap device now get the bridge name stored
    in the ActualNetDef, we can just always use
    virDomainNetGetActualBridgeName() instead.