ide: remove undefined behavior in ide-test

trivial: initialize the dirty buffer with a random-ish byte.
Stops valgrind from whining about uninitialized buffers.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
