tools: specify datadir for qemu-xen build to fix firmware loading

tools: specify datadir for qemu-xen build to fix firmware loading

qemu-xen does currently not find the firmware files, such as
vgabios-cirrus.bin. The reason is that qemu-xen uses the default prefix
/usr/local. Use SHAREDIR/qemu-xen as directory so that it can coexist
with qemu-traditional which is installed in SHAREDIR/xen/qemu.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>