ppc: Fix bug in handling of PAPR hypercall exits

Currently for powerpc, kvm_arch_handle_exit() always returns 1, meaning
that its caller - kvm_cpu_exec() - will always exit immediately afterwards
to the loop in qemu_kvm_cpu_thread_fn().

There's no need to do this.  Once we've handled the hypercall there's no
reason we can't go straight around and KVM_RUN again, which is what ret = 0
will signal.  The only exception might be for hypercalls which affect the
state of cpu_can_run(), however the only one that might do this is H_CEDE
and for kvm that is always handled in the kernel, not qemu.

Furtherm setting ret = 0 means that when exit_requested is set from a
hypercall, we will enter KVM_RUN once more with a signal which lets the
the kernel do its internal logic to complete the hypercall with out
actually executing any more guest code.  This is important if our hypercall
also triggered a reset, which previously would re-initialize everything
without completing the hypercall.  This caused the kernel to get confused
because it thought the guest was still in the middle of a hypercall when
it has actually been reset.

This patch therefore changes to ret = 0, which is both a bugfix and a small
optimization.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
