Include btrfs volumes names/labels in DeviceTreeBase.names

Btrfs Volumes use label of the filesystem as name in blivet, which
isn't really a name, so it doesn't have to be unique, but not
having it unique causes problems in both Blivet and Anaconda where
we assume device name is a unique identifier. Not having the name
in the list of names means newly created devices can get the same
name as an already existing (or scheduled to be created) device.

blivet/devicetree.py

@@ -151,7 +151,6 @@ def names(self):
         for dev in self._devices + self._hidden:
             # don't include "req%d" partition names
             if (dev.type != "partition" or not dev.name.startswith("req")) and \
-               dev.type != "btrfs volume" and \
                dev.name not in names:
                 names.append(dev.name)