pvgrub: ignore NUL

When using pvgrub in graphical mode with vnc, the grub timeout doesn't
work: the countdown doesn't even start. With a serial terminal the
problem doesn't occur and the countdown works as expected.

It turns out that the problem is that when using a graphical terminal,
checkkey () returns 0 instead of -1 when there is no activity on the
mouse or keyboard. As a consequence grub thinks that the user typed
something and interrupts the count down.

To fix the issue simply ignore keystrokes returning 0, that is the NUL
character anyway. Add a patch to grub.patches to do that.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Steven Haigh <netwiz@crc.id.au>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

stubdom/grub.patches/11graphics-keyboard.diff

@@ -0,0 +1,13 @@
+diff --git a/stage2/stage2.c b/stage2/stage2.c
+index 9d9fcc3..8353a3b 100644
+--- a/stage2/stage2.c
++++ b/stage2/stage2.c
+@@ -395,7 +395,7 @@ restart:
+ 	 pressed.  
+ 	 This avoids polling (relevant in the grub-shell and later on
+ 	 in grub if interrupt driven I/O is done).  */
+-      if (checkkey () >= 0 || grub_timeout < 0)
++      if (checkkey () > 0 || grub_timeout < 0)
+ 	{
+ 	  /* Key was pressed, show which entry is selected before GETKEY,
+ 	     since we're comming in here also on GRUB_TIMEOUT == -1 and