Fallback implicit partition size must be big enough for BTRFS

(Current) minimum BTRFS volume size is 256 MiB.

Related: rhbz#1202877
(cherry picked from commit 16eace7)

Conflicts:
	blivet/devices.py

blivet/devices/partition.py

@@ -45,7 +45,7 @@
 DEFAULT_PART_SIZE = Size("500MiB")
 
 # in case the default partition size doesn't fit
-FALLBACK_DEFAULT_PART_SIZE = Size("10MiB")
+FALLBACK_DEFAULT_PART_SIZE = Size("256MiB")
 
 class PartitionDevice(StorageDevice):
     """ A disk partition.