qapi: Move context-free checking to the proper place

QAPISchemaCommand.check() and QAPISchemaEvent().check() check 'data'
is present when 'boxed': true.  That's context-free.  Move to
check_command() and check_event().

Tweak the error message while there.

check_exprs() & friends now check exactly what qapi-code-gen.txt calls
the second layer of syntax.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-18-armbru@redhat.com>
2 files changed