Controller type "USB 3" doesn't boot.

[Sam Varshavchik]

This one has stumped me for a while, before I figured out the problem. I  
have two guests, on two different hosts. Both of them are qemu 4.1.1, and I  
use libvirt/virt-manager.

For some reason, one of them stubbornly refused to boot off a USB thumb  
drive. In both cases I had no problem adding the USB device in virtmanager.  
It appears normally, in the list of bootable devices. I can enable it as a  
bootable device and set it to be the first in the boot order. So far so good.

Both qemu hosts behave identically in this fashion. But one of them  
completely ignores the USB device when booting it.

And after the guest (Win 10) finishes booting off its usual "IDE disk", the  
guest sees the USB thumb drive without any issues. I can move the same thumb  
drive to the other host, go through the same steps, and the guest boots off  
the same thumb drive just fine.

After comparing the two hosts I discovered that the problematic one had its  
USB Controller showing as a "USB 3" controller type, in virt-manager. I  
changed it to "USB 2", and this fixed everything. The thumb drive showed up  
in the boot menu, and I can now boot off it.

I captured the qemu command line, for both configurations, and diffed it.  
Here are the only differences in the command line options to qemu:

--- qemu3       2020-02-03 19:59:14.512674308 -0500
+++ qemu2       2020-02-03 20:00:13.044800981 -0500
@@ -5,7 +5,7 @@
guest=WINDOWS10,debug-threads=on
-S
-object
-secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-3-WINDOWS10/master-key.aes
+secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-4-WINDOWS10/master-key.aes
-machine
pc-i440fx-1.4,accel=kvm,usb=off,dump-guest-core=off
-cpu
@@ -30,7 +30,13 @@
-boot
menu=on,strict=on
-device
-qemu-xhci,p2=15,p3=15,id=usb,bus=pci.0,addr=0x6
+ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x6.0x7
+-device
+ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x6
+-device
+ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x6.0x1
+-device
+ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x6.0x2
-device
virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x7
-drive

"-" is the non-working configuration with a USB 3 controller, and "+" is the  
working configuration with a USB 2 controller. I'm just curious as  to what  
the underlying issue is, here.

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