migration: export migration_is_running

Delete the MigrationState parameter from migration_is_running and move
it to the public API in misc.h.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Link: https://lore.kernel.org/r/1710179338-294359-5-git-send-email-steven.sistare@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>

system/dirtylimit.c

@@ -451,7 +451,7 @@ static bool dirtylimit_is_allowed(void)
 {
     MigrationState *ms = migrate_get_current();
 
-    if (migration_is_running(ms->state) &&
+    if (migration_is_running() &&
         (!qemu_thread_is_self(&ms->thread)) &&
         migrate_dirty_limit() &&
         dirtylimit_in_service()) {