docs/interop/firmware: Add 'uefi-vars' member for FirmwareMappingMemory

docs/interop/firmware: Add 'uefi-vars' member for FirmwareMappingMemory

This allows describing firmwares that are loaded as ROMs but also
support a UEFI variable store. This is the case for edk2 builds
that are set up to use the uefi-vars QEMU device, and whose
descriptors would advertise the 'host-uefi-vars' feature.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260217131553.313772-2-abologna@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>