guestperf: Support deferred migration for multifd

guestperf: Support deferred migration for multifd

The way to enable multifd migration has been changed by commit,
82137e6c8c (migration: enforce multifd and postcopy preempt to
be set before incoming), and guestperf has not made the
necessary changes. If multifd migration had been enabled in the
previous manner, the following error would have occurred:
Multifd must be set before incoming starts

Supporting deferred migration will fix it.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <8874e170f890ce0bc6f25cb0d9b9ae307ce2e070.1739530098.git.yong.huang@smartx.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>