ppc: fix ppc_set_compat() with KVM PR

When running in KVM PR mode, kvmppc_set_compat() always fail because the
current PR implementation doesn't handle KVM_REG_PPC_ARCH_COMPAT. Now that
the machine code inconditionally calls ppc_set_compat_all() at reset time
to restore the compat mode default value (commit 66d5c492dd3a9), it is
impossible to start a guest with PR:

qemu-system-ppc64: Unable to set CPU compatibility mode in KVM:
 Invalid argument

A tentative patch [1] was recently sent by Suraj to address the issue, but
it would prevent the compat mode to be turned off on reset. And we really
don't want to explicitely check for KVM PR. During the patch's review,
David suggested that we should only call the KVM ioctl() if the compat
PVR changes. This allows at least to run with KVM PR, provided no compat
mode is requested from the command line (which should be the case when
running PR nested). This is what this patch does.

While here, we also fix the side effect where KVM would fail but we would
change the CPU state in QEMU anyway.

[1] http://patchwork.ozlabs.org/patch/782039/

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
