XenAPI: Fix declare definition

Some versions of bash do not initialise a variable with declare -a
therefore with set -u bash can terminate the script.

Be more verbose in declaring the array if it is not set

Change-Id: I6ec2b6e986aeffe539a2ab93432fa7af9e5a4f5d
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/80/110280/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch