pc: Require xen when initializing xenfv machine

Without this check, the xen-platform device will crash on reset
if using the accel option with anything other than xen (e.g.
"-machine xenfv,accel=kvm").
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

hw/i386/pc_piix.c

@@ -434,6 +434,11 @@ static void pc_xen_hvm_init(MachineState *machine)
 {
     PCIBus *bus;
 
+    if (!xen_enabled()) {
+        error_report("xenfv machine requires the xen accelerator");
+        exit(1);
+    }
+
     pc_xen_hvm_init_pci(machine);
 
     bus = pci_find_primary_bus();