[libvirt] [RFC] Should libvirt be a proxy for migration data

Hi all, Currently when we start a non-tunneled migration, data go straight from source qemu to destination qemu. This is nice in that there is no additional overhead but it also has several disadvantages. If the communication between source and destination qemu breaks, we only get unexpected error message from qemu with no glue about what happened. Another issue is that if qemu cannot send migration data, we cannot cancel the migration because migrate_cancel blocks until all buffers with migration data queued up for transmission are written into the socket. That said, I think we should act as a proxy between source and destination qemu so that we can detect and report normal errors (such as connection reset by peer) and cancel migration at any time. Since we have virNetSocket and we already use that for connecting to destination qemu, we should use it for proxying migration data as well. This approach also has some disadvantages, e.g., a single libvirt thread instead of several qemu processes will now send migration data from all domains that are being migrated. However, I feel like the gain is bigger than the downside. And we already do the same for tunneled migration anyway. Any objections? Jirka