def vmid_check(var, val):
    val = int(val)
    if val <= 0:
        raise ValueError
    return val

xm_vars.var('vmid',
            use="Virtual machine id. Integer greater than 0.",
            check=vmid_check)

xm_vars.check()
kernel = "/boot/vmlinuz-2.6-xen"
ramdisk = "/boot/initrd-xen-3.0.2-2.img"
memory = 128
name = "VM%d" % vmid
vcpus = 2 # make your domain a 4-way
vif = [ '' ]
disk = [ 'phy:hdc3,sda1,w' ]
dhcp="dhcp"
root = "/dev/hdc3"
extra = "3 VMID=%d usr=/dev/hdc3" % vmid