engine: host name is <UNKNOWN> in event log when extend fails

When extend fails, an event log shows the following error:
Failed to connect Host <UNKNOWN> to device.

This patch assure host name is seen instead of '<UNKNOWN>'.

Change-Id: Id9076b78f4ae2e39a60ddb41448b3cc1e794aa02
Bug-Url: https://bugzilla.redhat.com/873251
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>