Use just name as device ID for multipath devices (#2327619)

Anaconda relies on disks using name as device ID because it needs
to be able to process the disks from kickstart without using
blivet. For this use case multipath devices are basically disks so
we should just use name here as well.

blivet/devices/disk.py

@@ -254,6 +254,10 @@ def __init__(self, name, fmt=None, size=None, wwn=None,
                           exists=True)
         self.wwn = wwn or None
 
+    @property
+    def device_id(self):
+        return self.name
+
     @property
     def model(self):
         if not self.parents: