Below is some guideline when perform patching on the Host machine which running Hyper V roles:-
a) Backup all VM before patching.
b) Properly shutdown all the VM before patching. Do not "Save" the VM
c) After patching and if you encounter problem such as cannot start the VM. Just delete the Virtual Machine and create a new Virtual Machine with using existing vhd.
Apply Patch KB950050 before apply Service Pack 2 (applicable for host machine running operating system with Pre-Hyper V version.
Be careful when patch the Hyper V machine...