fix raw_pread_aligned return value

raw_pread_aligned currently returns the raw return value from
lseek/read, which is always -1 in case of an error.  But the
callers higher up the stack expect it to return the negated
errno just like raw_pwrite_aligned.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block/raw-posix.c

@@ -249,7 +249,7 @@ static int raw_pread_aligned(BlockDriverState *bs, int64_t offset,
 
 label__raw_read__success:
 
-    return ret;
+    return  (ret < 0) ? -errno : ret;
 }
 
 /*