migration/multifd: Cleanup src flushes on condition check

migration/multifd: Cleanup src flushes on condition check

The src flush condition check is over complicated, and it's getting more
out of control if postcopy will be involved.

In general, we have two modes to do the sync: legacy or modern ways.
Legacy uses per-section flush, modern uses per-round flush.

Mapped-ram always uses the modern, which is per-round.

Introduce two helpers, which can greatly simplify the code, and hopefully
make it readable again.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241206224755.1108686-7-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>