qemu: Add range checking for scheduler tunables when changed by API

qemu: Add range checking for scheduler tunables when changed by API

The quota and period tunables for cpu scheduler accept only a certain
range of values. When changing the live configuration invalid values get
rejected. This check is not performed when changing persistent config.

This patch adds a separate range check, that improves error messages
when changing live config and adds the check for persistent config.
This check is done only when using the API. It is still possible to
specify invalid values in the XML.