Description of problem:
Shortly after dom0 becomes loaded (eg. installing a domU), I lose USB interrupts
with the kernel message:

rq 16: nobody cared (try booting with the "irqpoll" option)

Call Trace: <ffffffff80153b30>{__report_bad_irq+48}
       <ffffffff80153d47>{note_interrupt+458} <ffffffff801535f2>{__do_IRQ+191}
       <ffffffff8011060a>{do_IRQ+65} <ffffffff8025f818>{evtchn_do_upcall+123}
       <ffffffff8010e665>{do_hypervisor_callback+17}
<ffffffff8010a0ca>{hypercall_page+202}
       <ffffffff8010a0ca>{hypercall_page+202} <ffffffff8010c005>{xen_idle+124}
       <ffffffff8010c35c>{cpu_idle+255} <ffffffff804b47f5>{start_kernel+497}
       <ffffffff804b4211>{_sinittext+529}
handlers:
[<ffffffff80295c43>] (usb_hcd_irq+0x0/0x50)
[<ffffffff80295c43>] (usb_hcd_irq+0x0/0x50)
Disabling IRQ #16

This effectively kills USB.

Version-Release number of selected component (if applicable):
2.6.15-1.1955_FC5hypervisor on x86_64

How reproducible:
100%

Steps to Reproduce:
1. Boot 
2. Do something (eg. install a domU)
3. Wait a few minutes

Additional info:
Was seen on 1.1948 too.  Hardware is dual HT x86_64 Intel CPUs.  The presence of
the same USB irq handler twice on the handler list is suspicious.