tests: Boot PowerPC using PMU instead of CUDA for power management

tests: Boot PowerPC using PMU instead of CUDA for power management

A recent refactoring of CUDA command code has exposed a bug in OpenBIOS [1]
which was causing system powerdown and system reset to fail, thus causing
the QEMU instance to hang. This in turn caused the grub-shell command to
timeout causing it to return an error code when the test actually completed
successfully.

Since it could be a while before the patch fixing this issue in OpenBIOS
filters down to the average distro, switch to PMU to allow powerdowns and
reboots to work as expected.

[1] https://gitlab.com/qemu-project/qemu/-/issues/624

Signed-off-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>