tools/libxc: report malloc errors in writev_exact

The caller of writev_exact should be notified about malloc errors
when dealing with partial writes.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>
(cherry picked from commit 0d8d289af7)

tools/libxc/xc_private.c

@@ -722,6 +722,7 @@ int writev_exact(int fd, const struct iovec *iov, int iovcnt)
                     if ( !local_iov )
                     {
                         saved_errno = ENOMEM;
+                        rc = -1;
                         goto out;
                     }