The spec for ID_DFR0_EL1 says "In an AArch64-only implementation,
this register is UNKNOWN." Indeed ThunderX just returns zero when
that register is read. This means we can't rely on a non-zero
value to determine if we can test the PMU. For AArch64 we need to
read ID_AA64DFR0_EL1. This patch has the side effect of no longer
running PMU tests on TCG for AArch64. That's actually another fix,
though, as TCG chooses not to implement a PMU for AArch64 at this
time. The only way it worked before was probing the wrong register
and proceeding even though the version was 2, which is not a valid
version for AArch64. When TCG eventually implements a PMU things
should "just work".

Reviewed-by: Wei Huang <wei@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>