qemu: Ignore temporary job errors when checking migration status
    
    When qemu driver is polling for migration to finish (in
    qemuMigrationWaitForCompletion), it may happen that another job allowed
    during migration is running and if it does not finish within 30 seconds,
    migration would be cancelled because of that. However, we can just
    ignore the timeout and let the waiting loop try again later.
    
    If an event fired at the end of migration is ever implemented in QEMU,
    we can just wait for the event instead of polling for migration status
    and libvirt will behave consistently, i.e., migration won't be cancelled
    in case another job started during migration takes long time to finish.
    
    For bug https://bugzilla.redhat.com/show_bug.cgi?id=1083238
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>