qemu: Drop KVM assignment

qemu: Drop KVM assignment

KVM style of PCI devices assignment was dropped in kernel in
favor of vfio pci (see kernel commit v4.12-rc1~68^2~65). Since
vfio is around for quite some time now and is far superior
discourage people in using KVM style.

Ideally, I'd make QEMU_CAPS_VFIO_PCI implicitly assumed but turns
out qemu-3.0.0 doesn't support vfio-pci device for RISC-V.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>