system/dirtylimit: Don't use migration_is_active()

vcpu_dirty_rate_stat_collect() uses migration_is_active() to detect
whether migration is running or not, in order to get the correct dirty
rate period value.

However, recently there has been an effort to simplify the migration
status API and reduce it to a single migration_is_running() function.

To accommodate this, and since the same functionality can be achieved
with migration_is_running(), use it instead of migration_is_active().

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Hyman Huang <yong.huang@smartx.com>
Tested-by: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20241218134022.21264-6-avihaih@nvidia.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

system/dirtylimit.c

@@ -80,8 +80,7 @@ static void vcpu_dirty_rate_stat_collect(void)
     int i = 0;
     int64_t period = DIRTYLIMIT_CALC_TIME_MS;
 
-    if (migrate_dirty_limit() &&
-        migration_is_active()) {
+    if (migrate_dirty_limit() && migration_is_running()) {
         period = migrate_vcpu_dirty_limit_period();
     }