vm.py: Fix unsupported HPET option in ppc64

This patch does not allow disabling HPET in ppc64, since the -no-hpet
option is missing in the ppc64 QEMU.

Change-Id: I6df40fad6c6d70e5006a0b3bf2ca3660f8c75404
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Reviewed-on: http://gerrit.ovirt.org/24332
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>