xenapi:live-block-migration - fix XML client test

Fixes bug 1161356

KeyError: 'OS-EXT-SRV-ATTR:host' was raised while testing the live block
migration. This patch overrides the host detection in the XML client
case.

Change-Id: I90638d5b4a908171472611c4da5c5d915979ea6a
Merge "xenapi:live-block-migration - fix XML client test"