avocado/boot_linux_console.py: check TCG accel in test_ppc_mac99()

avocado/boot_linux_console.py: check TCG accel in test_ppc_mac99()

This test breaks when run in an IBM POWER host with a QEMU compiled
with --disable-tcg and the ppc-softmmu target in a similar manner as
test_ppc_g3beige did.

There's also an observation made about kvm_pr in the error message:

Command: ./qemu-system-ppc -display none -vga none (...)
-machine mac99 (...)
        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

This means that, when/if we're able to detect kvm_pr support in these
avocado tests, we can revisit this test to not rely solely on TCG
availability.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220310183011.110391-3-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>