qemu: support use of stateless EFI firmware

qemu: support use of stateless EFI firmware

When the <loader stateless='yes'/> attribute is set, the QEMU driver
needs to do three things

 - Avoid looking for an NVRAM template
 - Avoid auto-populating an <nvram/> path
 - Find firmware descriptors with mode=stateless instead of mode=split

Note, the first thing happens automatically when we solve the second
thing.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>