Remove XML support from v3 API plugins

Remove XML support from the following v3 compute API plugins:

 - hosts
 - hypervisors
 - instance_actions
 - ips
 - keypairs

Part of blueprint remove-v3-xml-api
DocImpact

Change-Id: I49da82632c884cdac36ee37674a4d0fc360fdd83
Merge "Remove XML support from v3 API plugins"