Here is how to do it. Its quite simple....
xen_vm vm;
xen_vbd_set *vbds;
xen_vbd_record *vbdr;
xen_vdi vdi;
xen_vdi_record *vdir;
enum xen_vbd_type vbd_type;
xen_vm_get_by_uuid(XEN.session,&vm,vm_uuid);
xen_vm_get_vbds(XEN.session, &vbds, vm);
for(int i=0;i<vbds->size;i++)
{
xen_vbd_get_record(XEN.session, &vbdr, vbds->contents[i]);
xen_vbd_get_type(XEN.session, &vbd_type, vbds->contents[i]);
if(vbd_type==XEN_VBD_TYPE_DISK)
{
xen_vbd_get_vdi(XEN.session, &vdi, vbds->contents[i]);
xen_vdi_get_record(XEN.session, &vdir, vdi);
printf("%"PRId64"bytes\n",vdir->virtual_size);
xen_vdi_record_free(vdir);
xen_vdi_free(vdi);
}
xen_vbd_record_free(vbdr);
}
xen_vbd_set_free(vbds);
xen_vm_free (vm);