qapi: Improve reporting of invalid 'if' errors

qapi: Improve reporting of invalid 'if' errors

Move check_if() from check_keys() to check_exprs() and call it later,
so its error messages gain an "in definition" line.

Checking values in a function named check_keys() is unclean anyway.
The original sin was commit 0545f6b887 "qapi: Better error messages
for bad expressions", which checks the value of key 'name'.  More
sinning in commit 2cbf09925a "qapi: More rigorous checking for type
safety bypass", commit c818408e44 "qapi: Implement boxed types for
commands/events", and commit 967c885108 "qapi: add 'if' to top-level
expressions".  This commit does penance for the latter.  The next
commits will do penance for the others.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-19-armbru@redhat.com>