qcow2: Use unsigned int for both members of Qcow2COWRegion

Qcow2COWRegion has two attributes:

- The offset of the COW region from the start of the first cluster
  touched by the I/O request. Since it's always going to be positive
  and the maximum request size is at most INT_MAX, we can use a
  regular unsigned int to store this offset.

- The size of the COW region in bytes. This is guaranteed to be >= 0,
  so we should use an unsigned type instead.

In x86_64 this reduces the size of Qcow2COWRegion from 16 to 8 bytes.
It will also help keep some assertions simpler now that we know that
there are no negative numbers.

The prototype of do_perform_cow() is also updated to reflect these
changes.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -406,8 +406,8 @@ int qcow2_encrypt_sectors(BDRVQcow2State *s, int64_t sector_num,
 static int coroutine_fn do_perform_cow(BlockDriverState *bs,
                                        uint64_t src_cluster_offset,
                                        uint64_t cluster_offset,
-                                       int offset_in_cluster,
-                                       int bytes)
+                                       unsigned offset_in_cluster,
+                                       unsigned bytes)
 {
     BDRVQcow2State *s = bs->opaque;
     QEMUIOVector qiov;

block/qcow2.h

@@ -301,10 +301,10 @@ typedef struct Qcow2COWRegion {
      * Offset of the COW region in bytes from the start of the first cluster
      * touched by the request.
      */
-    uint64_t    offset;
+    unsigned    offset;
 
     /** Number of bytes to copy */
-    int         nb_bytes;
+    unsigned    nb_bytes;
 } Qcow2COWRegion;
 
 /**