block/vpc: fix uninitialised variable compiler warning

Since commit cfc87e0 "block/vpc.c: Handle write failures in
get_image_offset()" older versions of gcc (in this case 4.7) incorrectly
warn that "ret" can be used uninitialised in vpc_co_pwritev().

Setting ret to 0 at the start of vpc_co_pwritev() prevents the warning
in gcc 4.7 and enables compilation with -Werror to succeed.

Signed-off-by: Mark Cave-Ayland <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: [email protected]
Signed-off-by: Peter Maydell <[email protected]>

block/vpc.c

@@ -649,7 +649,7 @@ vpc_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
     int64_t image_offset;
     int64_t n_bytes;
     int64_t bytes_done = 0;
-    int ret;
+    int ret = 0;
     VHDFooter *footer =  (VHDFooter *) s->footer_buf;
     QEMUIOVector local_qiov;