util/iov: introduce qemu_iovec_init_extended

Introduce new initialization API, to create requests with padding. Will
be used in the following patch. New API uses qemu_iovec_init_buf if
resulting io vector has only one element, to avoid extra allocations.
So, we need to update qemu_iovec_destroy to support destroying such
QIOVs.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190604161514.262241-2-vsementsov@virtuozzo.com
Message-Id: <20190604161514.262241-2-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>