tools: enable QEMU for ARM builds

tools: enable QEMU for ARM builds

Build qemu-xen on ARM and ARM64: it is used to provide the PV backends,
disk and framebuffer in particular.

Ideally we would also modify the configure options to only build what is
necessary: a machine just for PV backends. However that is a work in
progress and not yet available in QEMU (see
http://marc.info/?l=qemu-devel&m=139082425718379&w=2). So we just build
the usual i386 target, even though no i386 emulation is going to be done
by qemu-xen on ARM.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>