conf: support stateless UEFI firmware

conf: support stateless UEFI firmware

Normally when an UEFI firmware is marked as read-only, an associated
NVRAM file will be created. Some builds of UEFI firmware, however, wish
to remain stateless and so will be read-only, but never have any NVRAM
file. To represent this concept a 'stateless' tristate bool attribute
is introduced on the <loader/> element.

There are rather a large number of permutations to consider.

With default firmware selection

  *  <os/>

     => Historic default, no change

  *  <os>
       <loader stateless='yes'/>
     </os>

     => Explicit version of historic default, no change

  *  <os>
       <loader stateless='no'/>
     </os>

      => Invalid, bios is always stateless

With manual legacy BIOS selection

  *  <os>
       <loader>/path/to/seabios</loader>
       ...
     </os>

     => Historic default, no change

  *  <os>
       <loader stateless='yes'>/path/to/seabios</loader>
       ...
     </os>

     => Explicit version of historic default, no change

  *  <os>
       <loader stateless='no'>/path/to/seabios</loader>
       ...
     </os>

      => Invalid, bios is always stateless

With manual UEFI selection

  *  <os>
       <loader type='pflash'>/path/to/edk2</loader>
       ...
     </os>

     => Historic default, no change

  *  <os>
       <loader type='pflash' stateless='yes'>/path/to/edk2</loader>
       ...
     </os>

     => Skip auto-filling NVRAM / template

  *  <os>
       <loader type='pflash' stateless='no'>/path/to/edk2</loader>
       ...
     </os>

     => Explicit version of historic default, no change

With automatic firmware selection

  *  <os firmware='bios'/>

     => Historic default, no change

  *  <os firmware='bios'>
       <loader stateless='yes'/>
     </os>

     => Explicit version of historic default, no change

  *  <os firmware='bios'>
       <loader stateless='no'/>
     </os>

      => Invalid, bios is always stateless

  *  <os firmware='uefi'/>

     => Historic default, no change

  *  <os firmware='uefi'>
       <loader stateless='yes'/>
     </os>

     => Skip auto-filling NVRAM / template

  *  <os firmware='uefi'>
       <loader stateless='no'/>
     </os>

     => Explicit version of historic default, no change

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