tests: Use new guestfs_add_drive_scratch API where possible in tests.

Replaces code such as:

  fd = open "test1.img"
  ftruncate fd, size
  close fd
  g.add_drive "test1.img"

with the shorter and simpler:

  g.add_drive_scratch size

haskell/Guestfs050LVCreate.hs

@@ -24,6 +24,8 @@ import Control.Monad
 
 main = do
   g <- G.create
+  {- XXX replace with a call to add_drive_scratch once
+         optional arguments are supported -}
   fd <- openFile "test.img" WriteMode
   hSetFileSize fd (500 * 1024 * 1024)
   hClose fd