vhost: add traces for memory listeners

Trace these operations on two memory listeners.  It helps to verify the
new memory listener fix, and good to keep them there.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180122060244.29368-2-peterx@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>