Add schema for attachments
Add schema for attachments

Looks like:

  <attachment href="/storagedomains/1234/attachments/1111">
    <status>ACTIVE</status>
    <storage_domain>
      <id>1234</id>
      <link rel="self" href="/storagedomains/1234"/>
    </storage_domain>
    <datacenter>
      <id>4567</id>
      <link rel="self" href="/datacenters/4567"/>
    </datacenter>
  </attachment>

markmc/rhevm-api,colloquium/rhevm-api,markmc/rhevm-api,colloquium/rhevm-api,markmc/rhevm-api