I'm getting the same bug as reported at https://github.com/QubesOS/qubes-issues/issues/2018
The bug was originally reported against r3.1 and fedora-23-minimal, and fixed in r3.2 and fedora-24-minimal. However, I'm getting it on r3.2, using fedora-26 (full, not minimal) for both my USB VM and my appVM. Both Qubes and the template are fully updated. Rebooting the system doesn't fix it. Trying a different appVM also doesn't fix it.
qvm-block -l says the USB device is attached to the appVM. But the appVM has no /dev/xvdi
xl block-list shows the appVM's four standard block devices with state 4, and it shows a fifth device with state 3. This is the same result that the OP got. I don't know what the state numbers 3 and 4 mean, and neither does google. The man page for xl also says nothing about them.
/var/log/xen/xen-hotplug.log reveals nothing helpful.
The OP solved his problem by installing perl in the template. But I'm using the full fedora-26, which already has perl installed.