qcow2: Use better type for numerical snapshot ID

When trying to find a new snapshot ID, the existing ones are converted
to integers using strtoul. This function returns an unsigned long,
therefore its result should be saved in an unsigned long as well.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-snapshot.c

@@ -292,7 +292,8 @@ static void find_new_snapshot_id(BlockDriverState *bs,
 {
     BDRVQcowState *s = bs->opaque;
     QCowSnapshot *sn;
-    int i, id, id_max = 0;
+    int i;
+    unsigned long id, id_max = 0;
 
     for(i = 0; i < s->nb_snapshots; i++) {
         sn = s->snapshots + i;
@@ -300,7 +301,7 @@ static void find_new_snapshot_id(BlockDriverState *bs,
         if (id > id_max)
             id_max = id;
     }
-    snprintf(id_str, id_str_size, "%d", id_max + 1);
+    snprintf(id_str, id_str_size, "%lu", id_max + 1);
 }
 
 static int find_snapshot_by_id_and_name(BlockDriverState *bs,