qtest: Fix rtas dependencies

qtest: Fix rtas dependencies

qtest "rtas" command is only available with pseries not all ppc64 targets,
so if I try to compile only powernv machine, the build fails with:

  /usr/bin/ld: qtest.o: in function `qtest_process_command':
  .../qtest.c:645: undefined reference to `qtest_rtas_call'

We fix this by enabling rtas command only with pseries machine.

Fixes: eeddd59f5962 ("tests: add RTAS command in the protocol")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200205232016.588202-2-lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>