A VM that changes its memory faster than the network carries it does
not finish a live migration without auto converge (measured: a guest
rewriting about 1.2 GB/s did not finish in 90 s over an 8.5 Gbit/s
link, and finished in 16-18 s with it). Auto converge only slows the
VM's CPUs while the migration cannot keep up, so a calm VM is not
affected.
The migrate form ticks it for a running VM. The API already documented
it as on by default but read a left-out option as off; the serializer
now has the defaults (auto converge on, compress and unsafe off) for
JSON and form-encoded requests, and the schema shows them.
