vmci: simplify qp_dequeue_locked()

* no need for callback argument - it's always the same one
* fold __qp_memcpy_from_queue() into its only caller, get rid of dead code
* pass struct iov_iter * without casting to void *
* don't pass buf_size at all - it's always iov_iter_count(to)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>