Comment # 4 on bug 1202851 from Larry Finger

Unfortunately, the machine log did not show much. The only thing is that your
VM log has

00:05:58.179284 xHCI: USB Suspended
00:05:58.182952 ACPI: Reset initiated by ACPI
00:05:58.183013 Changing the VM state from 'RUNNING' to 'RESETTING'

Mine shows

00:01:13.598452 OHCI: Software reset
00:01:13.679583 ACPI: Entering S5 power state (power down)
00:01:13.679638 Changing the VM state from 'RUNNING' to 'POWERING_OFF'

I did a normal shutdown from the desktop. My system is failing to mount USB3
sticks on a USB3 port, which is why I have OHCI rather than xHCI.

I do not understand why you are getting a reset rather than a power down.