To make a USB Host Device optional on startup within KVM, edit the XML and add startupPolicy="optional" to the source node. This will ensure your usb stick, yubikey, or whatever, it won’t stop your VM from booting if it isn’t present.
Example:
<hostdev mode="subsystem" type="usb" managed="yes">
<source startupPolicy="optional">
<vendor id="0x1050"/>
<product id="0x0402"/>
</source>
<address type="usb" bus="0" port="4"/>
</hostdev>Code language: HTML, XML (xml)