The following USB configuration works for usb passthrough of a USB 3.0 flash drive to Fedora 25 guest.
-device nec-usb-xhci,id=xhci \
-device usb-host,bus=xhci.0,vendorid=0x0781,productid=0x5590 \
-usb -device usb-mouse,bus=usb-bus.0 -device usb-kbd,bus=usb-bus.0 \
...
The following USB configuration works for usb passthrough of a Moto G3 phone to Fedora 25 guest.
-device usb-host,bus=usb-bus.0,vendorid=0x22b8,productid=0x002e \
-usb -device usb-mouse,bus=usb-bus.0 -device usb-kbd,bus=usb-bus.0 \
...
The following USB configuration works for usb passthrough of a CoolerMaster keyboard to macOS Sierra guest!
-device usb-host,bus=usb-bus.0,vendorid=0x2516,productid=0x0004 \
-usb -device usb-tablet,bus=usb-bus.0 -device usb-kbd,bus=usb-bus.0 \
...
The following USB configuration works for attaching a virtual USB disk to macOS Sierra guest. Use "qemu-img" to create "disk.raw" virtual disk.
-drive if=none,id=usbstick,file=disk.raw,format=raw \
-device usb-storage,bus=usb-bus.0,drive=usbstick \
...
However USB passthrough of EHCI, and XHCI (USB 3.0) devices does not work with macOS Sierra. See https://bugs.launchpad.net/qemu/+bug/1509336 for confirmation. According to this bug report, USB passthrough does not work with versions >= Mac OS X El Capitan guests.
It seems that this problem can be fixed by using OVMF + Clover.
Update: OVMF + Clover doesn't help. It seems that macOS is missing the required drivers for the EHCI, and XHCI controllers that are exposed by QEMU.