tests/migration: fix parameter of auto-converge migration

tests/migration: fix parameter of auto-converge migration

when execute the following test command:
$ ./guestperf-batch.py --auto-converge \
    --auto-converge-step {percent} ...
test aborts and error message be throwed as the following:
"Parameter 'x-cpu-throttle-increment' is unexpected"

The reason is that 'x-cpu-throttle-increment' has been
deprecated and 'cpu-throttle-increment' was introduced
Since v2.7. Use the new parameter instead.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Message-Id: <0195d34a317ce3cc417b3efd275e30cad35a7618.1616513998.git.huangy81@chinatelecom.cn>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>