qemu: support firmware descriptor flash 'mode' for optional NVRAM

qemu: support firmware descriptor flash 'mode' for optional NVRAM

Currently the 'nvram_template' entry is mandatory when parsing the
firmware descriptor based on flash. QEMU is extending the firmware
descriptor spec to make the 'nvram_template' optional, depending
on the value of a new 'mode' field:

  - "split"
      * "executable" contains read-only CODE
      * "nvram_template" contains read-write VARS

  - "combined"
      * "executable" contains read-write CODE and VARs
      * "nvram_template" not present

  - "stateless"
      * "executable" contains read-only CODE and VARs
      * "nvram_template" not present

In the latter case, the guest OS can write vars but the
firmware will make no attempt to persist them, so any changes
will be lost at poweroff.

For now we parse this new 'mode' but discard any firmware
which is not 'mode=split' when matching for a domain.

In the tests we have a mixture of files with and without the
mode attribute.

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