Remove unused methods from VirtAPI

The instance_get_all_by_host() and instance_get_by_uuid() methods are
no longer needed by any virt drivers, so remove them from the virtapi..

Related to blueprint unified-object-model

Change-Id: Ib51bf3b3e068d69e7a75f7312f03db6c85e7cf49
Merge "Remove unused methods from VirtAPI"