gnome-boxes --checks was reporting
Libvirt KVM guest available: no
while KVM support was otherwise functional in GNOME-Boxes.
Turns out it's a bug in the XPath expression check_libvirt_kvm uses,
here is the XML we want to match:

<capabilities>
  <guest>
    <os_type>hvm</os_type>
    <arch name='x86_64'>
      <emulator>/usr/bin/qemu-system-x86_64</emulator>
      <domain type='qemu'/>
    </arch>
  </guest>
</capabilities>

The current XPath expression expects 'emulator' to be a child of
'domain'.