Atualizei minha versão e o problema não ocorreu mais!
Veja o que Cole Robinson me respondeu na lista de discussão "libvirt-users" ([email protected]):
Coincidentally this a bug I fixed just this morning:
commit b1eb07c26aae1b303cd9893427f46f5b693544b3 (HEAD -> master,
origin/master, origin/HEAD)
Author: Cole Robinson <[email protected]>
Date: Wed Aug 22 08:38:33 2018 -0400
guest: Don't add usb tablet for xen PV
xen PV doesn't have USB support
But you can work around it: at the end of the 'new vm' wizard, click
'customize before install', and in the VM details window, remove the
tablet device, then finish the install. That should make things work
Obrigado! = D
OBSERVAÇÃO: link