util/iov: add iov_discard_undo()

util/iov: add iov_discard_undo()

The iov_discard_front/back() operations are useful for parsing iovecs
but they modify the array elements. If the original array is needed
after parsing finishes there is currently no way to restore it.

Although g_memdup() can be used before performing destructive
iov_discard_front/back() operations, this is inefficient.

Introduce iov_discard_undo() to restore the array to the state prior to
an iov_discard_front/back() operation.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Li Qiang <[email protected]>
Message-Id: <20200917094455[email protected]>