Libvirt driver: normalize variable names (part1)

Variable names referring to the same objects/data are different
throughout the libvirt driver. This can lead to confusion, which
leads to avoidable bugs.

Part1:
    ctxt -> context
    instance_ref -> instance

Change-Id: I9d170b6025935544f07ecdebe5a68b2e2eaba5dd
Merge "Libvirt driver: normalize variable names (part1)"