Use kvm acceleration if available.
Disable kernel-irqchip and use qemu64 cpu
for both kvm and tcg cases.

Using kvm acceleration saves about a second
and disabling kernel-irqchip has no visible
performance impact.
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>