This change implements the actual functionality to allow users to
launch instances with stateless firmware (read-only firmware image +
no NVRAM).
Note that this feature is supported by the libvirt virt driver, and
also requires libvirt >= 8.6.0.
Implements: blueprint libvirt-stateless-firmware
Change-Id: I7219bfa11ae98e65c326bec1a99c49d3e245cb9a