Add unit tests for libvirt domain creation

Tests for:
nova.virt.libvirt.driver.LibvirtDriver.get_guest_config
nova.virt.libvirt.driver.LibvirtDriver._create_domain

Partially implements: bp libvirt-lxc-user-namespaces

Change-Id: I4aada9ac06cd423b04a4c6136165c65e64e378aa
Merge "Add unit tests for libvirt domain creation"