xend: Do not mess with bridge if admin has set one up already

xend: Do not mess with bridge if admin has set one up already

Previously, the default "network-script",
/etc/xen/scripts/network-bridge, would attempt to do its horrid work
even if you had already set everything up in /etc/network/interfaces.

Setting up your bridge in /etc/network/interfaces is:
 * easy
 * required for libxl since libxl never does it for you
 * not a fragile piece of lunacy
 * properly documented
 * the way everyone would expect it to work

In this small patch we make it so that the default config for xend
doesn't mess about on startup if you already have a bridge, and
doesn't mess about on shutdown unless your first-named bridge (eth0 or
xenbr0, normally) doesn't also have a physical interface named
p<whatever> (peth0 or pxenbr0) enslaved to it.  The latter test is not
ideal but will hopefully do from now until the time xend finally dies.

We also fix the "documentation" - ie, the comments in the default
xend-config.sxp - to correspond to reality.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>