qemu_file: Fix mismerge of "use fwrite() correctly"

Reviewers accepted v2 of the patch, but what got committed was v1,
with the R-bys for v2.  This is the v1->v2 followup fix.

[Amit:
 This fixes commit aded6539d9
]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

qemu-file.c

@@ -105,7 +105,7 @@ static int stdio_put_buffer(void *opaque, const uint8_t *buf, int64_t pos,
     res = fwrite(buf, 1, size, s->stdio_file);
 
     if (res != size) {
-        return -EIO;	/* fake errno value */
+        return -errno;
     }
     return res;
 }