lxc: always use virDomainNetGetActualBridgeName to get interface's bridge

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.