conf, schema: add 'id' field for cells

conf, schema: add 'id' field for cells

In XML format, by definition, order of fields should not matter, so
order of parsing the elements doesn't affect the end result.  When
specifying guest NUMA cells, we depend only on the order of the 'cell'
elements.  With this patch all older domain XMLs are parsed as before,
but with the 'id' attribute they are parsed and formatted according to
that field.  This will be useful when we have tuning settings for
particular guest NUMA node.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>