Use the list when get information from libvirt

If the libvirt adds new elements to the domain info struct or others,
when use the new libvirt, there will be a valueError (too many values
to unpack). When we get information from libvirt, we should use the
original variable format. This change is for compatibility with upcoming
versions of libvirt which change this signature.

Change-Id: I0271a260a53fe8f5fc17b2934ebe3a3c9ee0c130
Closes-Bug: #1291805
Merge "Use the list when get information from libvirt"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch