conf: pass elementName to virDomainThreadSchedParseHelper

conf: pass elementName to virDomainThreadSchedParseHelper

Pass the scheduler element name instead of trying to reconstructing
it from the attribute name.

This has the benefit of not mixing '%s' with regular text in
translatable strings as well as preventing the confusion when
the 's' marking the plural in the element name ('vcpus') is taken
as a first letter of the 'sched' suffix.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 7ea55a481dd45f09b54425005362533ecc800cd2
Fixes: 99c5fe0e7c26c08103415248ffef1f5acb81ddc7
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>