Dell T430 - Xen PCI Passthrough not working

I am using Xen 4.6 with CentOS kernel 3.18.34-20.el7.x86_64 on a Dell Poweredge T430.
I am having trouble getting PCI Passthrough to work from Dom0 running CentOS 7 to DomU runnning Debian 8. It seems to be an IRQ problem with the Dom0(Host).
I think I have set it all up correctly, but I see no message when putting a USB device into any of the USB slots on the Guest
In the Dom0(Host) I get this in syslog when booting the DomU(Guest), even with irqpoll in the Guest kernel line

xen_pciback: xen-pciback[0000:00:1a.0] IRQ line is not shared with other domains. Turning ISR off
Jul 03 11:31:23 myDom0.mynetwork.net kernel: irq 18: nobody cared (try booting with the "irqpoll" option)
Jul 03 11:31:23 myDom0.mynetwork.net kernel: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.18.34-20.el7.x86_64 #1
Jul 03 11:31:23 myDom0.mynetwork.net kernel: Hardware name: Dell Inc. PowerEdge T430/0975F3, BIOS 1.5.4 10/05/2015
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  0000000000000000 ffff8803bc603d88 ffffffff81653783 ffff8803b223e400
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  ffff8803b223e48c ffff8803bc603db8 ffffffff810c6776 ffff8803bc613340
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  ffff8803b223e400 0000000000000012 0000000000000000 ffff8803bc603e08
Jul 03 11:31:23 myDom0.mynetwork.net kernel: Call Trace:
Jul 03 11:31:23 myDom0.mynetwork.net kernel:    [ ] dump_stack+0x64/0x82
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] __report_bad_irq+0x36/0xd0
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] note_interrupt+0x226/0x270
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? add_interrupt_randomness+0x3c/0x1f0
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] handle_irq_event_percpu+0xcc/0x1e0
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] handle_irq_event+0x3b/0x60

Message from syslogd@myDom0 at Jul  3 11:31:23 ...
 kernelisabling IRQ #18
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] handle_fasteoi_irq+0x7a/0x130
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] generic_handle_irq+0x2b/0x40
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] evtchn_fifo_handle_events+0x162/0x170
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] __xen_evtchn_do_upcall+0x50/0x90
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] xen_evtchn_do_upcall+0x37/0x50
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] xen_do_hypervisor_callback+0x1e/0x40
Jul 03 11:31:23 myDom0.mynetwork.net kernel:    [ ] ? xen_hypercall_sched_op+0xa/0x20
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? xen_hypercall_sched_op+0xa/0x20
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? xen_safe_halt+0x10/0x20
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? default_idle+0x24/0xf0
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? arch_cpu_idle+0xf/0x20
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? cpu_startup_entry+0x312/0x3e0
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? rest_init+0x77/0x80
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? start_kernel+0x4d0/0x4dd
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? set_init_arg+0x55/0x55
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? x86_64_start_reservations+0x2a/0x2c
Jul 03 11:31:23 myDom0.mynetwork.net kernel:  [ ] ? xen_start_kernel+0x5a9/0x5b5
Jul 03 11:31:23 myDom0.mynetwork.net kernel: handlers:
Jul 03 11:31:23 myDom0.mynetwork.net kernel: [ ] usb_hcd_irq
Jul 03 11:31:23 myDom0.mynetwork.net kernel: [ ] xen_pcibk_guest_interrupt [xen_pciback]
Jul 03 11:31:23 myDom0.mynetwork.net kernel: Disabling IRQ #18

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Is there some setting I need to enable or disable in the BIOS for this to work?

Many thanks

Francis