tools: require at least pixman 0.21.8 for qemu-xen

tools: require at least pixman 0.21.8 for qemu-xen

Avoid late build failure in openSUSE 11.4, it has just pixman-0.20:

....
[  211s] ERROR: pixman >= 0.21.8 not present. Your options:
[  211s]          (1) Preferred: Install the pixman devel package (any recent
[  211s]              distro should have packages as Xorg needs pixman too).
[  211s]          (2) Fetch the pixman submodule, using:
[  211s]              git submodule update --init pixman
....

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>