migration: Add property x-postcopy-preempt-break-huge

Add a property field that can conditionally disable the "break sending huge
page" behavior in postcopy preemption.  By default it's enabled.

It should only be used for debugging purposes, and we should never remove
the "x-" prefix.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Manish Mishra <manish.mishra@nutanix.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220707185511.27366-1-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>