vmci: fix buf_size in case of iovec-based accesses

Both qp_dequeue_locked() and qp_enqueue_locked() use the
buf_size argument to decide how much would be there to copy;
in case of iovec- (== msghdr-)based primitives it's not
iov_size, it's msg_data_left(msg).

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