tools/configure: Check if pixman is present on the system when building QEMU

tools/configure: Check if pixman is present on the system when building QEMU

QEMU upstream requires the use of pixman. When pixman is not present the
system, the configure of QEMU will fail with:

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

I think we can use by default the version on the system. So check it
a Xen configuration time to avoid a build issue later.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- reran autogen.sh, adjusted comment ]