When a host goes becomes unavailable, the server logs the following message:

... temporarily disabling Zabbix agent checks on host "testhost": host unavailable

And if the host comes back server logs:
... enabling Zabbix agent checks on host "testhost": host became available

It would be useful, if the actions based on the event source "Internal" would allow to setup an eventype " Host in unavailable state" as a condition similar to the existing eventtype "Item in not supported state"