Yes that is most likely your issue.
In theory I may have a work around for you‎. Although I have never tried it for this purpose.
This depends on your card a bit if you have a higher end USB3 PCI card then it will have an individual root hub for each port. In that case each one will show up in lspci.
If that is the case then you can try mapping the PCI address to the VM that might make it bootable for the VM with some low lever tinkering directly in the config.
The other thing is are you using a GUI to manage the config, if so you may find that this is an arbitrary limitation in the GUI and not in KVM it self. In that case you may be able to get it to work via the config file.

  Original Message  
From: Konstantin Olchanski
Sent: Tuesday, May 10, 2016 20:41
To: ToddAndMargo
Cc: [log in to unmask]
Subject: Re: Anyone know of a bootable USB3 PCI or PCIe card?

On Tue, May 10, 2016 at 05:29:13PM -0700, ToddAndMargo wrote:
> 
> Any of you guys know of a bootable PCIe USB adapter
> that is bootable?
> 


Never seen such a thing. Had similar problem with non-bootable PCI SATA and GigE interfaces.

Not bootable because machine BIOS is too old and does not have a "driver"
for the newer chip on your board. So in order to be bootable, the PCI card would
have to have an EPROM/flash chip that contains the required BIOS bits.

This makes a bootable USB interface should be easy to identify by photograph -
in addition to the PCI<->USB bridge chip, it will have one more chip
for the BIOS.


-- 
Konstantin Olchanski
Data Acquisition Systems: The Bytes Must Flow!
Email: olchansk-at-triumf-dot-ca
Snail mail: 4004 Wesbrook Mall, TRIUMF, Vancouver, B.C., V6T 2A3, Canada