Adds return-type in two functions' docstrings

Change-Id: Ifdf49c59d8e983905ad6f31e0a3f7f64cd5a297c
Fixes: bug #1130922
NoneType exception thrown if driver live-migration check returns None

XenAPI driver live-migration checks must return dicts as required by
the virt api interface.

Resolves bug 1158603.

Change-Id: I0db364a17e3069b0b7e3f19030afc35283edecfb
Merge "NoneType exception thrown if driver live-migration check returns None"
Merge "Adds return-type in two functions' docstrings"