When dynamic Redfish emulator:
* is running against libvirt backend and
* node boot mode is being set and
* boot loader path is not empty
* <os><loader> element is not present in libvirt domain
configuration
Then create <os><loader> element in `readOnly` mode (to
force libvirt using a private copy of the loader for environment
variables). If boot loader path is empty, then remove
<os><loader> element if it was present at all.
The rationale is to ensure well-formed domain configuration
upon boot mode change.
Change-Id: Id434be3287ab158a25e8c091ec23be281da29915
caa0593a9c