virtio-net: add Max MTU configuration field It is helpful for a host to indicate it's MTU to be set on guest NICs other than the assumed 1500 byte value. This helps in situations where the host network is using Jumbo Frames, or aiding in PMTU discovery by configuring a homogenous network. It is also helpful for sizing receive buffers correctly. The change adds a new field to configuration area of network devices. It will be used to pass a maximum MTU from the device to the driver. This will be used by the driver as a maximum value for packet sizes during transmission, without segmentation offloading. In addition, in order to support backward and forward compatibility, we introduce a new feature bit called VIRTIO_NET_F_MTU. git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio/trunk@585 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652