NoneType exception thrown if driver live-migration check returns None

Multiple bugs have been reported for different drivers about a change
in behaviour of the compute manager live-migration checks that now
requires drivers to return a dict instead of just passing None as the
return value. See bug 1129091 (Hyper-V) and bug 1158603 (XenAPI).

Resolves bug 1158603.

Change-Id: I0db364a17e3069b0b7e3f19030afc35283edecfb