xenapi: tidy ups to some spawn related methods

There were server methods using during spawn that are only used
internally, but not named that way. They have been renamed in this
change.

There were also several methods that are currently called in a single
place, but had logic around the call that should have been inside that
method. This has been tided up, so it will be easier to re-use and/or
move those calls in future refactoring.

This is one of the building blocks towards sharing more code between
spawn and finish_migration.

Change-Id: I424daf8334ad9904572b9df511d8e6bc1ff6d21f
Merge "xenapi: tidy ups to some spawn related methods"