migration: Fix cleanup of iochannel in file migration

migration: Fix cleanup of iochannel in file migration

The QIOChannelFile object already has its reference decremented by
g_autoptr. Trying to unref an extra time causes:

ERROR:../qom/object.c:1241:object_unref: assertion failed: (obj->ref > 0)

Fixes: a701c03dec ("migration: Drop reference to QIOChannel if file seeking fails")
Fixes: 6d3279655a ("migration: Fix file migration with fdset")
Reported-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>