qapi/expr.py: Modify check_keys to accept any Collection

qapi/expr.py: Modify check_keys to accept any Collection

This is a minor adjustment that lets parameters @required and
@optional take tuple arguments, in particular ().  Later patches will
make use of that.

(Iterable would also have worked, but Iterable also includes things like
generator expressions which are consumed upon iteration, which would
require a rewrite to make sure that each input was only traversed
once. Collection implies the "can re-iterate" property.)

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210421182032.3521476-10-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>