Add a _create method to BlivetDiskVolume.

Relying on _reformat to create the formatting worked because a
DiskDevice always exists. Now that self._device can be an
optional, non-existent, LUKS layer on top of the disk we have to
make the disk volume class behave more like the other volume
classes -- namely, it has to create its format as part of _create
since we will not always call _reformat (eg: when we've set up,
but not yet created, a new LUKS layer on the disk).

library/blivet.py

@@ -357,6 +357,9 @@ def _get_device_id(self):
     def _type_check(self):
         return self._device.raw_device.is_disk
 
+    def _create(self):
+        self._reformat()
+
     def _look_up_device(self):
         super(BlivetDiskVolume, self)._look_up_device()
         if not self._get_device_id():