restapi: Add "size" support to VMs

This patch adds support for new "size" detail specifications for the VMs
collection.

When the "size" detail is requested the result will look like thiss:

  GET /vms;detail=+size HTTP/1.1
  All-Content: true

  <vms>
    <size>123</size>
    <vm id="..." href="...">
      ...
    </vm>
    ...
  </vms>

The "main" detail is intended to suppress output of the actual data, so that
only the size will be populated:

  GET /vms;detail=-main+size HTTP/1.1
  All-Content: true

  <vms>
    <size>123</size>
  </vms>

Change-Id: Ic089745a03e7f01d4c96e9267c0ff084bd913e0d
Bug-Url: https://bugzilla.redhat.com/1132247
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>