qemu: always use virDomainNetGetActualBridgeName to get interface's bridge
    
    qemuNetworkIfaceConnect() 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.
    
    (an audit of the two callers to qemuNetworkIfaceConnect() confirms
    that it is never called for any other type of network, so the dead
    code in the else statement (logging an internal error if it is called
    for any other type of network) is eliminated in the process.)