vhost-user-bridge: fix iov_restore_front() warning

vhost-user-bridge: fix iov_restore_front() warning

  CC      tests/vhost-user-bridge.o
/home/dgilbert/git/qemu-world3/tests/vhost-user-bridge.c:228:23: warning: variables 'front' and 'iov' used in loop condition not modified in loop body [-Wfor-loop-analysis]
    for (cur = front; front != iov; cur++) {
                      ^~~~~    ~~~
1 warning generated.

Fix the loop, document the function, and fix some related assert().

In practice, the loop bug was harmless because the front sg buffer is
enough to discard/restore the header size.

Reported-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Tested-by: Jens Freimann <[email protected]>