nbd-server: fix setup_serve() to report errors

nbd-server: fix setup_serve() to report errors

This commit changes setup_serve() to report its errors via GError object
to its caller. It also add resource deallocation code to return points
to free all local resources correctly. Note that, the previous version
of the setup_serve() did not free its resources properly on error, but
just called functions which terminated the process. This was techincally
a leak, but without any practical significance due to immediate process
termination. However, now that error handling (process termination) is
separeted from error reporting, we must deallocate/free all locally
allocated resources correctly on errors.

Signed-off-by: Tuomas Räsänen <tuomasjjrasanen@tjjr.fi>