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Hit this problem on rhel6.7.

Host:
qemu-img-0.12.1.2-2.469.el6.x86_64
gpxe-roms-qemu-0.9.7-6.14.el6.noarch
qemu-kvm-debuginfo-0.12.1.2-2.469.el6.x86_64
qemu-kvm-tools-0.12.1.2-2.469.el6.x86_64
qemu-kvm-0.12.1.2-2.469.el6.x86_64
2.6.32-556.el6.x86_64

# /usr/libexec/qemu-kvm -monitor stdio -device ich9-usb-ehci1,id=ehci0 -device usb-host,hostbus=1,hostaddr=31,id=usb-mouse,bus=ehci0.0 -qmp tcp::8888,server,nowait
VNC server running on `::1:5900'
husb: open device 1.31
husb: grabbed usb device 1.31
qemu-kvm: Warning: speed mismatch trying to attach usb device USB Optical Mouse to bus ehci0.0

QEMU 0.12.1 monitor - type 'help' for more information
(qemu) husb: open device 1.31
husb: grabbed usb device 1.31
qemu-kvm: Warning: speed mismatch trying to attach usb device USB Optical Mouse to bus ehci0.0

husb: open device 1.31
husb: grabbed usb device 1.31
qemu-kvm: Warning: speed mismatch trying to attach usb device USB Optical Mouse to bus ehci0.0

Not going to be fixed in RHEL-6, we don't have the error handling infrastructure needed.  Simply exiting unconditinally isn't going to fly because one can hotplug usb devices.