To do the conversion, the file_backend_class_init() was moved
after the getter/setter functions. The old
file_backend_instance_init() function was removed because it is
not needed anymore.

The NULL errp arguments on the property registration calls were
changed to &error_abort.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>