vm: replace minidom.Element for memory snapshots

The _memorySnapshot method used to manipulate directly
xml.dom.minidom.Element items.
This patch allow the code to do the equivalent by using
vmxml.Element instead.

Change-Id: I2b0ba3b102da7b2687adaa31b6dd222b13f214ad
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29695
Reviewed-by: Dan Kenigsberg <danken@redhat.com>