qom: Use returned bool to check for failure, manual part

The previous commit used Coccinelle to convert from checking the Error
object to checking the return value.  Convert a few more manually.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200707160613.848843-30-armbru@redhat.com>
3 files changed