qemu-img: Fix assert when mapping unaligned raw file

Commit a290f085 exposed a latent bug in qemu-img map introduced
during the conversion of block status to be byte-based.  Earlier in
commit 5e344dd8, the internal interface get_block_status() switched
to take byte-based parameters, but still called a sector-based
block layer function; as such, rounding was added in the lone
caller to obey the contract.  However, commit 237d78f8 changed
get_block_status() to truly be byte-based, at which point rounding
to sector boundaries can result in calling bdrv_block_status() with
'bytes == 0' (a coding error) when the boundary between data and a
hole falls mid-sector (true for the past-EOF implicit hole present
in POSIX files).  Fix things by removing the rounding that is now
no longer necessary.

See also https://bugzilla.redhat.com/1589738

Fixes: 237d78f8
Reported-by: Dan Kenigsberg <danken@redhat.com>
Reported-by: Nir Soffer <nsoffer@redhat.com>
Reported-by: Maor Lipchuk <mlipchuk@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -2906,7 +2906,7 @@ static int img_map(int argc, char **argv)
         int64_t n;
 
         /* Probe up to 1 GiB at a time.  */
-        n = QEMU_ALIGN_DOWN(MIN(1 << 30, length - offset), BDRV_SECTOR_SIZE);
+        n = MIN(1 << 30, length - offset);
         ret = get_block_status(bs, offset, n, &next);
 
         if (ret < 0) {