managedsave/start appears to work, but VM is unusable due to non-stop
repeating uhci_hcd errors.

I can't remove the USB controller from QEMU, but I CAN remove it
through virsh edit.  Doing so eliminates the USB controller in the
guest, preventing non-stop repeating uhci_hcd errors.  (QEMU only lets
you remove the USB redirection entries, which didn't help.)

Using libvirt 1.2.18 (-1 Arch) and QEMU git-master (2.4.0.r40384.2d69736).

Installed using Q35 chipset.

I'm running QEMU git, which allows SCSI controller migration, so I can
attempt doing this.

==========
virsh # managedsave <vmname>

Domain <vmname> state saved by libvirt

virsh # start <vmname>
Domain <vmname> started

{{{ Then, viewing the VM console through QEMU, its screen is extremely
laggy, taking about a minute to fill up the screen with repeating
errors within 0.003 kernel seconds: }}}
uhci_hcd 0000:02:02.2: host system error, PCI problems?
uhci_hcd 0000:02:02.2: host controller process error, something bad happened!
{{{ repeats }}}
==========
{{{ after a forced reboot }}}
$ dmesg|grep 0000:02:02
[    0.480054] pci 0000:02:02.0: [8086:2934] type 00 class 0x0c0300
[    0.498451] pci 0000:02:02.0: reg 0x20: [io  0xc0e0-0xc0ff]
[    0.506965] pci 0000:02:02.1: [8086:2935] type 00 class 0x0c0300
[    0.525101] pci 0000:02:02.1: reg 0x20: [io  0xc100-0xc11f]
[    0.533634] pci 0000:02:02.2: [8086:2936] type 00 class 0x0c0300
[    0.554992] pci 0000:02:02.2: reg 0x20: [io  0xc120-0xc13f]
[    0.563642] pci 0000:02:02.7: [8086:293a] type 00 class 0x0c0320
[    0.566679] pci 0000:02:02.7: reg 0x10: [mem 0xfc041000-0xfc041fff]
[    1.720668] ehci-pci 0000:02:02.7: EHCI Host Controller
[    1.720709] ehci-pci 0000:02:02.7: new USB bus registered, assigned
bus number 1
[    1.721343] ehci-pci 0000:02:02.7: irq 22, io mem 0xfc041000
[    1.730198] ehci-pci 0000:02:02.7: USB 2.0 started, EHCI 1.00
[    1.740641] uhci_hcd 0000:02:02.0: UHCI Host Controller
[    1.740667] uhci_hcd 0000:02:02.0: new USB bus registered, assigned
bus number 2
[    1.740813] uhci_hcd 0000:02:02.0: detected 2 ports
[    1.741026] uhci_hcd 0000:02:02.0: irq 23, io base 0x0000c0e0
[    1.747517] uhci_hcd 0000:02:02.1: UHCI Host Controller
[    1.747543] uhci_hcd 0000:02:02.1: new USB bus registered, assigned
bus number 3
[    1.747664] uhci_hcd 0000:02:02.1: detected 2 ports
[    1.747904] uhci_hcd 0000:02:02.1: irq 20, io base 0x0000c100
[    1.753055] uhci_hcd 0000:02:02.2: UHCI Host Controller
[    1.753074] uhci_hcd 0000:02:02.2: new USB bus registered, assigned
bus number 4
[    1.753624] uhci_hcd 0000:02:02.2: detected 2 ports
[    1.753724] uhci_hcd 0000:02:02.2: irq 21, io base 0x0000c120
==========