virt: use context & instance as param names in migrate APIs

The vast majority of methods in the ComputeDriver use
'context' and 'instance' as parameter names. The only
exceptions are the migration related methods which use
'ctx' and 'instance_ref'. Change the latter to be
consistently named with the rest of the file

Partial-bug: #1333219
Change-Id: I0457e3e70ee58a9cbfc69f8cfe675de0504cc781
Merge "virt: use context & instance as param names in migrate APIs"