rbd nbd: move server startup code to function

rbd nbd: move server startup code to function

The netlink connect cmd interface requires the server to be running when
the connect call completes or commands will timeout/fail. This breaks the
server code to 2 functions so we can control when it starts and runs.

Right now it looks odd in that start_server will allocate and return
the server only for it be freed after run, and it is not touched. The
netlink enablement patch at the end will use the returned server.

Signed-off-by: Mike Christie <mchristi@redhat.com>