Change VhostUserSlaveReqHandler::set_slave_req_fd() to take an UnixStream instead of SlaveFsCacheReq

[jiangliu]

Currently VhostUserSlaveReqHandler::set_slave_req_fd() accepts an SlaveFsCacheReq input parameter, that means the slave communication channel could only used to support virtio-fs map/unmap requests.
It should take an UnixStream as input, so the backend implementation makes the decision what service will be supported on the slave communication channel.

[jiangliu]

@slp @sboeuf This will be a breaking change, any suggestions?

[sboeuf]

Since this is a proposal to make the code more generic, I'm totally okay with it.