virttest.libvirt_xml.vm_xml: Add option to get_disk_resource

Add option to get_disk_resource to support extra option when
dumpxml.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1294 from waynesun09/virsh_domblklist

[PR Updated] virttest.libvirt_xml.vm_xml: Add option to get_disk_resource