Automate methods should not be able to modify readonly domains

https://bugzilla.redhat.com/show_bug.cgi?id=1142317

The $evm.instance_create, $evm.instance_delete and
$evm.instance_update methods now check if the domain
is editable before creating/updating/deleting
@mkanoor Since this is returning true or false the method name should be ```__editable_instance?```