engine: Changes in VdsStaticDAO

Since there is a unique constraint on host_name now, the method
getAllForHostname() doesn't make sense any more. Hence replaced
it with getByHostName() that returns a single object

Change-Id: I0075430a97f19238b93da1e199fa73a4584556d6
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>