xenapi plugins must target only Python 2.4 features

Add a comment documenting the restriction that our xenapi plugins must
target Python 2.4 features because of restrictions with the XenServer
dom0 userspace.

Change-Id: Ide130923b16c0555c3190cbd63ce97e18d18aae0
Drop max-complexity to 47

Now that we fixed the two worst offenders, drop the mccabe
max-complexity setting in flake8 to 47.

Add note explaining how to generate a list of functions that are too
'complex'.

Change-Id: I44000e9e95c457ae9000c428460b5825d8ad3f92
Merge "xenapi plugins must target only Python 2.4 features"
Merge "Drop max-complexity to 47"