avocado/ppc_74xx.py: check TCG accel for all tests

avocado/ppc_74xx.py: check TCG accel for all tests

All tests of this file, when running in an IBM POWER host and with
--disable-tcg, fail in a similar manner:

        Command: ./qemu-system-ppc -display none -vga none (...)
-cpu 7400 (...)
        Output: ioctl(KVM_CREATE_VM) failed: 22 Invalid argument
PPC KVM module is not loaded. Try modprobe kvm_pr.
qemu-system-ppc: failed to initialize kvm: Invalid argument

We don't have a way of telling which KVM module is loaded in a Power
host (kvm_hv or kvm_pr). For now let's make all the tests of this
file depend on TCG support.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220310183011.110391-6-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>