conf: Provide error on undefined iothreadsched entry

conf: Provide error on undefined iothreadsched entry

When commit id '6dfb4507' refactored where the iothreadsched data was
stored, the error message for when the virDomainIOThreadIDFind failed
to find an iothreadid ("iothreadsched attribute 'iothreads' uses
undefined iothread ids") was lost. This led to the possibility that
someone would try to use it, but receive the generic message "An error
occurred, but the cause is unknown".

This patch adds the error message back so that someone will know that
they have an invalid configuration.

Signed-off-by: John Ferlan <jferlan@redhat.com>