qapi: make 'if' condition strings simple identifiers

qapi: make 'if' condition strings simple identifiers

Change the 'if' condition strings to be C-agnostic. It will accept
'[A-Z][A-Z0-9_]*' identifiers. This allows to express configuration
conditions in other languages (Rust or Python for ex) or other more
suitable forms.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
Message-Id: <20210804083105.97531-11-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased with semantic conflict in redefined-event.json]
Signed-off-by: Markus Armbruster <armbru@redhat.com>