qga: flatten safe_open_or_create()

There is a bit too much nesting in the function, this can be simplified
a bit to improve readability.

This also helps with the following error handling changes.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220525144140.591926-5-marcandre.lureau@redhat.com>