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>
(cherry picked from commit 17bec9235bb0775cf8dec4103c167757ee8898f3)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

migration/options.c

@@ -555,7 +555,7 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
             return false;
         }
 
-        if (migrate_incoming_started()) {
+        if (!migrate_postcopy_preempt() && migrate_incoming_started()) {
             error_setg(errp,
                        "Postcopy preempt must be set before incoming starts");
             return false;
@@ -563,7 +563,7 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
     }
 
     if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) {
-        if (migrate_incoming_started()) {
+        if (!migrate_multifd() && migrate_incoming_started()) {
             error_setg(errp, "Multifd must be set before incoming starts");
             return false;
         }