Convert xenapi/vmops uses of instance_update to objects

This converts uses of the old instance_update() conductor method
to use the instance object in xenapi/vmops.

Related to blueprint virt-objects-juno

Change-Id: I5fab06934a7f3e7426631bb75eafe8b0d4b0806d
Merge "Convert xenapi/vmops uses of instance_update to objects"