Log Message:
Add support for "vifname" keys inside XenStore. Its value is the
interface name for the vif, e.g. xvif(4) for dom0, and xennet(4)
for domU.

This allows querying the XenStore for the vif names, rather than
hardcoding their syntax in vif-* scripts.

Add a xen-subr shell script that can contain customized functions, and
include it in the vif-ip/vif-bridge scripts.
Introduce xenstore_read_default that returns the value of a specific
key from XenStore, or the default specified value when key is absent.

Bump revision.

ok bouyer@.

See http://mail-index.netbsd.org/port-xen/2011/01/11/msg006405.html