Minor fixes to create_sparse_header().

1)  descriptorOffset is never set by callers, so just
hard-code it to 1 in the create_sparse_header method.
2)  inFileSectors and descriptorSize are both required
for create_sparse_header to operate properly.  If a
caller forgot one of them, None would be used and then
the struct.pack at the end would fail anyway.  Just make
them required arguments.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

VMDKstream.py

@@ -79,15 +79,17 @@ def __str__(self):
 ddb.virtualHWVersion = "7"'''
 
 
-def create_sparse_header(inFileSectors = None, descriptorOffset = 1,
-                         descriptorSize = None, gdOffset = 0xFFFFFFFFFFFFFFFF):
+def create_sparse_header(inFileSectors, descriptorSize,
+                         gdOffset = 0xFFFFFFFFFFFFFFFF):
     # While theoretically variable we set these based on current VMWare
     # convention
     grainSize = 128
     numGTEsPerGT = 512
     overHead = 128
     formatVersion = 3 # NOTE: Conflicts with VMWare docs - determined by trial/error
 
+    descriptorOffset = 1
+
     # The following are always fixed in the "stream-optimized" format we are
     # creating
     compressAlgorithm = 1