avocado/ppc_prep_40p.py: check TCG accel in all tests

avocado/ppc_prep_40p.py: check TCG accel in all tests

All tests in the file times out when running in an IBM POWER host and
--disable-tcg with an error like the following:

        Command: ./qemu-system-ppc -display none -vga none (...)
-machine 40p (...)
        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

Since we don't have a way to detect whether the host is running kvm_hv
or kvm_pr, skip all tests if TCG is not available.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220310183011.110391-9-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>