hub: add the support for hub own flow control Only when all other hub port's *peer* .can_receive() all return 1, the source hub port .can_receive() return 1. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com>