virtio: mergeable rx buffers
virtio: mergeable rx buffers

Negotiate MRG_RXBUF and support multi-iovec packet transmission to VM.
This basically enables Jumbo Ethernet frames.

Signed-off-by: Nikolay Nikolaev <5fb6ff14f2a3b931c007d120a649a55e86928f6a@virtualopensystems.com>

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