util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended

util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended

Make it more obvious, that filling qiov corresponds to qiov allocation,
which in turn corresponds to total_niov calculation, based on mid_niov
(not mid_len). Still add an assertion to show that there should be no
difference.

[Added mingw "error: 'mid_iov' may be used uninitialized in this
function" compiler error fix suggested by Vladimir.
--Stefan]

Reported-by: Coverity (CID 1405302)
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20190910090310[email protected]
Suggested-by: Peter Maydell <[email protected]>
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20190910090310[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
fixup! util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended