Fixes Nova API /os-hosts missing element "zone"

Now the HostIndexTemplate of Nova /os-hosts API is missing element "zone",
which will cause the XML output of list hosts missing the zone attribute.

Fixes bug: 1159055

Change-Id: I9940efcfd364c35b00cde4398d531d7ff56dc2b9
xenapi: Retrieve VM uuid from xenstore.

Fall back to retrieving the uuid from xenstore if
/sys/hypervisor/uuid isn't accessible.

Change-Id: I409079068d3102ff86a71431b29c1ce2e6fe8857
Fixes: bug #1157211
Merge "xenapi: Retrieve VM uuid from xenstore."
Merge "Fixes Nova API /os-hosts missing element "zone""
Merge remote-tracking branch 'm_src_repo/master' into build-temp