qapi: Improve reporting of missing / unknown definition keys

qapi: Improve reporting of missing / unknown definition keys

Have check_exprs() call check_keys() later, so its error messages gain
an "in definition" line.

Both check_keys() and check_name_is_str() check the definition's name
is a string.  Since check_keys() now runs after check_name_is_str()
rather than before, its check is dead.  Bury it.  Checking values in
check_keys() is unclean anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-21-armbru@redhat.com>