Add idmap to libvirt config

Allows Nova to configure libvirt domains with id map
elements. This allows user namespaces to be enabled
and configured.

Partially implements: bp libvirt-lxc-user-namespaces

Change-Id: I4dbbc5e265e2caa17e32ff40cb3c5f5bb51d6548
Merge "Add idmap to libvirt config"