The VIOT test does not always work under KVM on the virt machine:

  PASS 5 qtest-aarch64/bios-tables-test /aarch64/acpi/virt/oem-fields
  qemu-system-aarch64: kvm_init_vcpu: kvm_arch_init_vcpu failed (0): Invalid argument
  Broken pipe

Make it TCG only.

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>