Add libvirt XML schema support for resource tuning parameters

Extend the libvirt XML config classes to support for CPU, disk IO,
network traffic tuning parameters.

Blueprint: quota-instance-resource
DocImpact
original reviews https://review.openstack.org/#/c/14151/

Change-Id: I2827ea0e731fefe5bbaec6c385268a3f5b89f7f6
Merge "Add libvirt XML schema support for resource tuning parameters"