Remove redundant check for parents in Blivet.newBTRFS.

We already check parent type in BTRFSSubVolumeDevice, but we should be
using BTRFSValueError if those checks fail.

(cherry picked from commit 58884e4)

Resolves: rhbz#1072285

blivet/__init__.py

@@ -1223,11 +1223,6 @@ def newBTRFS(self, *args, **kwargs):
 
         if kwargs.pop("subvol", False):
             dev_class = BTRFSSubVolumeDevice
-            # make sure there's a valid parent device
-            parents = kwargs.get("parents", [])
-            if not parents or len(parents) != 1 or \
-               not isinstance(parents[0], BTRFSVolumeDevice):
-                raise ValueError("new btrfs subvols require a parent volume")
 
             # set up the subvol name, using mountpoint if necessary
             if not name:

blivet/devices/btrfs.py

@@ -434,10 +434,10 @@ def __init__(self, *args, **kwargs):
         super(BTRFSSubVolumeDevice, self).__init__(*args, **kwargs)
 
         if len(self.parents) != 1:
-            raise errors.DeviceError("%s %s must have exactly one parent." % (self.type, self.name))
+            raise errors.BTRFSValueError("%s %s must have exactly one parent." % self.type)
 
         if not isinstance(self.parents[0], BTRFSDevice):
-            raise errors.DeviceError("%s %s's unique parent must be a BTRFSDevice." % (self.type, self.name))
+            raise errors.BTRFSValueError("%s %s's unique parent must be a BTRFSDevice." % self.type)
 
         self.volume._addSubVolume(self)