Remove XML support from migrations pci multiple_create v3 API plugins

Remove XML support from the following v3 compute API plugins:

 - migrations
 - multiple_create
 - pci

Part of blueprint remove-v3-xml-api

DocImpact

Change-Id: If7dfda52eea63a39fae0e546cfeaf4b1d814b7b9
Merge "Remove XML support from migrations pci multiple_create v3 API plugins"