vtest: Refactor vtest_server.c

Refactor the main loop into smaller functions so that it is easier to extend with more functionality.

There should be no functional change with the exception of some socket listening code would return error codes as fds, where they now instead call perrno and exit(1).

Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

PS: Commit is currently marked as wip as it depends on !66 (closed) and targets that branch instead of master.