rdma: clean up of qemu_rdma_cleanup()

- It can't be determined by RDMAContext::cm_id != NULL if the connection
  is established or not.
- RDMAContext::cm_id is leaked and not destroyed because it is set to NULL
  too early.
- RDMAContext::qp is created by rdma_create_qp() so that it should be destroyed
  by rdma_destroy_qp(). not ibv_destroy_qp()

Cc: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Isaku Yamahata <yamahata@private.email.ne.jp>