python: Fix optargs so we don't use special sentinel values.

Previously with Python it was impossible to set a boolean or integer
optarg to -1 because that was used as a special sentinel value to
indicate that the optarg was not set.

Instead, use None as the sentinel value, since that cannot be a
boolean or integer type.

python/t/060-optargs.py

@@ -20,6 +20,6 @@ import guestfs
 
 g = guestfs.GuestFS()
 g.add_drive ("/dev/null")
-g.add_drive ("/dev/null", readonly = 1)
+g.add_drive ("/dev/null", readonly = True)
 g.add_drive ("/dev/null", iface = "virtio", format = "raw")
 g.close ()