vdsm: isolate VmDevice xml generation method

VmDevice class uses helper function 'createXmlElem' to ease generating
of XML for devices. This patch moves the function to xmldom in the form
of new parent class for VmDevice, separating the logic and xml
generation. (inspired by http://gerrit.ovirt.org/#/c/17694/)

Change-Id: I026917641da7e1a55b15ae3238ee545023cf129e
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29249
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>