avocado/ppc_405.py: check TCG accel in test_ppc_ref405ep()

Running this test without TCG support in an IBM POWER server results
in the following error:

        Command: ./qemu-system-ppc -display none -vga none (...)
-machine ref405ep (...)
        Output: qemu-system-ppc: Register sync failed... If you're using
kvm-hv.ko, only "-cpu host" is possible
qemu-system-ppc: kvm_init_vcpu: kvm_arch_init_vcpu failed (0): Invalid argument

Although the host is running kvm_hv we don't have a way of differentiate
between kvm_hv and kvm_pr, meaning that this test would've failed in the
same way if kvm_pr was the KVM module loaded in the host.

Since we don't have a way of checking which KVM module is being loaded
when using avocado, make a TCG accel check in test_ppc_ref405ep().

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220310183011.110391-5-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

tests/avocado/ppc_405.py

@@ -30,5 +30,7 @@ class Ppc405Machine(QemuSystemTest):
         :avocado: tags=arch:ppc
         :avocado: tags=machine:ref405ep
         :avocado: tags=cpu:405ep
+        :avocado: tags=accel:tcg
         """
+        self.require_accelerator("tcg")
         self.do_test_ppc405()