i386/xen: Initialize Xen backends from pc_basic_device_init() for emulation

Now that all the work is done to enable the PV backends to work without
actual Xen, instantiate the bus from pc_basic_device_init() for emulated
mode.

This allows us finally to launch an emulated Xen guest with PV disk.

   qemu-system-x86_64 -serial mon:stdio -M q35 -cpu host -display none \
     -m 1G -smp 2 -accel kvm,xen-version=0x4000a,kernel-irqchip=split \
     -kernel bzImage -append "console=ttyS0 root=/dev/xvda1" \
     -drive file=/var/lib/libvirt/images/fedora28.qcow2,if=none,id=disk \
     -device xen-disk,drive=disk,vdev=xvda

If we use -M pc instead of q35, we can even add an IDE disk and boot a
guest image normally through grub. But q35 gives us AHCI and that isn't
unplugged by the Xen magic, so the guests ends up seeing "both" disks.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

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