migration: Allow caps to be set when preempt or multifd cap enabled

migration: Allow caps to be set when preempt or multifd cap enabled

With commit 82137e6c8c ("migration: enforce multifd and postcopy preempt to
be set before incoming"), and if postcopy preempt / multifd is enabled, one
cannot setup any capability because these checks would always fail.

(qemu) migrate_set_capability xbzrle off
Error: Postcopy preempt must be set before incoming starts

To fix it, check existing cap and only raise an error if the specific cap
changed.

Fixes: 82137e6c8c ("migration: enforce multifd and postcopy preempt to be set before incoming")
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>