Create an actual TYPE_ACCEL object when initializing a machine. This
will allow accelerator classes to implement some initialization on
instance_init, and to save state on the TYPE_ACCEL object.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>