conf: Introduce VIR_DOMAIN_LOADER_TYPE_NONE

conf: Introduce VIR_DOMAIN_LOADER_TYPE_NONE

This is going to extend virDomainLoader enum. The reason is that
once loader path is NULL its type makes no sense. However, since
value of zero corresponds to VIR_DOMAIN_LOADER_TYPE_ROM the
following XML would be produced:

  <os>
    <loader type='rom'/>
    ...
  </os>

To solve this, introduce VIR_DOMAIN_LOADER_TYPE_NONE which would
correspond to value of zero and then use post parse callback to
set the default loader type to 'rom' if needed.

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