test: Make common test*ObjFindByName helpers

test: Make common test*ObjFindByName helpers

Rather than have continued repeated sequences of :

    testDriverLock()
    xxx = vir*ObjFindByName()
    testDriverUnlock()

    if (xxx == NULL) {
        virReportError
        goto cleanup;
    }

Make some common helpers which will use the pattern and make a single
reference using a single common error message.

Altered for Interfaces, Storage Pools, Storage Volumes, and Node Devices.

For each the common error message can now also indicate which 'name' was
not found. For Storage Volumes, the "new" error will be more specific
rather than just invalid argument.