Description of problem:
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Assign a usb 1.0 device to ehci controller, it fails to initialize, but occupies a port.

Version-Release number of selected component (if applicable):
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qemu-kvm-0.12.1.2-2.209.el6.x86_64

How reproducible:
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100%

Steps to Reproduce:
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1.boot guest with echi controller
2.assign usb-hub to ehci controller

(qemu) device_add usb-hub,id=hub1
Warning: speed mismatch trying to attach usb device QEMU USB Hub to bus ehci.0
Device 'usb-hub' could not be initialized
(qemu) info usb
  Device 0.0, Port 1, Speed 1.5 Mb/s, Product      <-- add this

  
Actual results:
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if we add 6 usb-hub with different port, the ehci controller has no port left, the 7th usb-hub causes core dump.

Expected results:
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Assign a usb 1.0 device to ehci controller, it fails to initialize, it should not occupy any port.