Change default xen root path to /dev/xvda1

All recent-enough versions of linux see the xen paravirtual device as
/dev/xvd*.

This doesn't break old installations, as the default is only used on new
clusters.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>