POWER9 DD1 silicon has some bugs which mean it a) isn't really compliant
with the ISA v3.00 and b) require a number of special workarounds in the
kernel.

At the moment, qemu isn't aware of DD1.  For TCG we don't really want it to
be (why bother emulating buggy silicon).  But with KVM, the guest does need
to be aware of DD1 so it can apply the necessary workarounds.

Meanwhile, the feature negotiation between qemu and the guest strongly
favours architected compatibility modes to "raw" CPU modes.  In combination
with the above, this means the guest sees architected POWER9 mode, and
doesn't apply the DD1 workarounds.  Well, unless it has yet another
workaround to partially ignore what qemu tells it.

This patch addresses this by disabling support for compatibility modes when
using KVM on a POWER9 DD1 host.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>