qapi: Replace List[str] with Sequence[str] for ifcond

qapi: Replace List[str] with Sequence[str] for ifcond

It does happen to be a list (as of now), but we can describe it in more
general terms with no loss in accuracy to allow tuples and other
constructs.

In the future, we can write "ifcond: Sequence[str] = ()" as a default
parameter, which we could not do safely with a Mutable type like a List.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-2-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>