Libvirt normalize numa cell ids

This patch normalizes the cell ids before writing xml.
As cell ids are used to store host cell association,
they need to be normalized before writing xml, or otherwise
the xml will be invalid.

Change-Id: I9b565670f5b0ad24c7a5ff1853648594643b4c7a
Closes-bug: #1397381
Merge "Libvirt normalize numa cell ids"