vhost-user-blk: Add support to reconnect backend

vhost-user-blk: Add support to reconnect backend

Since we now support the message VHOST_USER_GET_INFLIGHT_FD
and VHOST_USER_SET_INFLIGHT_FD. The backend is able to restart
safely because it can track inflight I/O in shared memory.
This patch allows qemu to reconnect the backend after
connection closed.

Signed-off-by: Xie Yongji <[email protected]>
Signed-off-by: Ni Xun <[email protected]>
Signed-off-by: Zhang Yu <[email protected]>
Message-Id: <20190320112646[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>