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>

hw/i386/pc.c

@@ -102,6 +102,11 @@
 #include "trace.h"
 #include CONFIG_DEVICES
 
+#ifdef CONFIG_XEN_EMU
+#include "hw/xen/xen-legacy-backend.h"
+#include "hw/xen/xen-bus.h"
+#endif
+
 /*
  * Helper for setting model-id for CPU models that changed model-id
  * depending on QEMU versions up to QEMU 2.4.
@@ -1318,6 +1323,8 @@ void pc_basic_device_init(struct PCMachineState *pcms,
         if (pcms->bus) {
             pci_create_simple(pcms->bus, -1, "xen-platform");
         }
+        xen_bus_init();
+        xen_be_init();
     }
 #endif