block/backup: avoid integer overflow of `max-workers`

QAPI generates `struct BackupPerf` where `max-workers` value is stored
in an `int64_t` variable.
But block_copy_async(), and the underlying code, uses an `int` parameter.

At the end that variable is used to initialize `max_busy_tasks` in
block/aio_task.c causing the following assertion failure if a value
greater than INT_MAX(2147483647) is used:

  ../block/aio_task.c:63: aio_task_pool_wait_one: Assertion `pool->busy_tasks > 0' failed.

Let's check that `max-workers` doesn't exceed INT_MAX and print an
error in that case.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2009310
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211005161157.282396-2-sgarzare@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block/backup.c

@@ -407,8 +407,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         return NULL;
     }
 
-    if (perf->max_workers < 1) {
-        error_setg(errp, "max-workers must be greater than zero");
+    if (perf->max_workers < 1 || perf->max_workers > INT_MAX) {
+        error_setg(errp, "max-workers must be between 1 and %d", INT_MAX);
         return NULL;
     }