vm: use vmxml.Element in snapshot method

Use vmxml.Element instead of xml.dom.minidom.Element
in vm.snapshot
No functional changes.

Change-Id: I938f7b65600ca43cc1d20d2ac32f1b7e3a4d50d9
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29697
Reviewed-by: Dan Kenigsberg <danken@redhat.com>