qmp-input: Don't consume input when checking has_member

qmp-input: Don't consume input when checking has_member

Commit e8316d7 mistakenly passed consume=true within
qmp_input_optional() when checking if an optional member was
present, but the mistake was silently ignored since the code
happily let us extract a member more than once.  Fix
qmp_input_optional() to not consume anything, then tighten up
the input visitor to ensure that a member is consumed exactly
once (all generated code follows this pattern; and the new
assert will catch any hand-written code that tries to visit
the same key more than once).

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1461879932[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>