target/user: Use iovec[] to describe continuous area

We don't need use one iovec per scatter-gather list entry, since data
area are continuous.

Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Sheng Yang <sheng@yasker.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>