virt-libvirt: virsh attach/detatch device test

This pull address testing of a highly variable virsh aspect: Attaching devices from XML snippets both while a VM is running, and not.  Different device types, host type, hardware, setup, and versions mean it's almost impossible to predict whether or not hot-attaching a device is supported or not.  This test deliberately assumes all devices can be both hot and cold attached, and it's up to the operator to filter tests for known support issues.  Additionally, both positive and negative testing is performed along with supporting testing with arbitrary device types and parameters.  This initial version includes two different types of serial devices, and one type of disk device for testing.