core: remove SearchReturnValue

SearchReturnValue extends VdcQueryReturnValue and adds the boolean
isSearchValid. However, this boolean is only set in various flows, and
never read, meaning this entire class is useless.

This patch does the following:
1. Removes SearchReturnValue
2. Removes QueriesCommandBase.createReturnValue() and replaces it with
   an inline call to new VdsQueryReturnValue(), since now there is no
   reason to override it.
3. Removes QueriesCommandBase.proceedOnFail(), since now no class
   implements it.

Change-Id: I39fd1f1fb4bf66e594ea3157bd36959736faa6dc
Signed-off-by: Allon Mureinik <amureini@redhat.com>