qemu-img: Speed up compare on pre-allocated larger file

Compare the following images with all-zero contents:
$ truncate --size 1M A
$ qemu-img create -f qcow2 -o preallocation=off B 1G
$ qemu-img create -f qcow2 -o preallocation=metadata C 1G

On my machine, the difference is noticeable for pre-patch speeds,
with more than an order of magnitude in difference caused by the
choice of preallocation in the qcow2 file:

$ time ./qemu-img compare -f raw -F qcow2 A B
Warning: Image size mismatch!
Images are identical.

real	0m0.014s
user	0m0.007s
sys	0m0.007s

$ time ./qemu-img compare -f raw -F qcow2 A C
Warning: Image size mismatch!
Images are identical.

real	0m0.341s
user	0m0.144s
sys	0m0.188s

Why? Because bdrv_is_allocated() returns false for image B but
true for image C, throwing away the fact that both images know
via lseek(SEEK_HOLE) that the entire image still reads as zero.
From there, qemu-img ends up calling bdrv_pread() for every byte
of the tail, instead of quickly looking for the next allocation.
The solution: use block_status instead of is_allocated, giving:

$ time ./qemu-img compare -f raw -F qcow2 A C
Warning: Image size mismatch!
Images are identical.

real	0m0.014s
user	0m0.011s
sys	0m0.003s

which is on par with the speeds for no pre-allocation.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1480,11 +1480,11 @@ static int img_compare(int argc, char **argv)
         while (sector_num < progress_base) {
             int64_t count;
 
-            ret = bdrv_is_allocated_above(blk_bs(blk_over), NULL,
+            ret = bdrv_block_status_above(blk_bs(blk_over), NULL,
                                           sector_num * BDRV_SECTOR_SIZE,
                                           (progress_base - sector_num) *
                                           BDRV_SECTOR_SIZE,
-                                          &count);
+                                          &count, NULL, NULL);
             if (ret < 0) {
                 ret = 3;
                 error_report("Sector allocation test failed for %s",
@@ -1492,11 +1492,11 @@ static int img_compare(int argc, char **argv)
                 goto out;
 
             }
-            /* TODO relax this once bdrv_is_allocated_above does not enforce
+            /* TODO relax this once bdrv_block_status_above does not enforce
              * sector alignment */
             assert(QEMU_IS_ALIGNED(count, BDRV_SECTOR_SIZE));
             nb_sectors = count >> BDRV_SECTOR_BITS;
-            if (ret) {
+            if (ret & BDRV_BLOCK_ALLOCATED && !(ret & BDRV_BLOCK_ZERO)) {
                 nb_sectors = MIN(nb_sectors, IO_BUF_SIZE >> BDRV_SECTOR_BITS);
                 ret = check_empty_sectors(blk_over, sector_num, nb_sectors,
                                           filename_over, buf1, quiet);