Revert "tests/migration: Reduce autoconverge initial bandwidth"

This reverts commit 6d1da867 ("tests/migration: Reduce autoconverge initial bandwidth")
since that change makes unit tests much slower for all developers, while it's not
a robust way to fix migration tests. Migration tests need to find
a more robust way to discover a reasonable bandwidth without slowing
things down for everyone.

Fixes: 6d1da867 ("tests/migration: Reduce autoconverge initial bandwidth")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>