BUG_ON() is a macro very very similar to assert(), and it's a really bad

BUG_ON() is a macro very very similar to assert(), and it's a really bad
idea in general to put statements with side-effects inside such a construct
(for example the BUG_ON() could be compiled away for non-debug builds).

The patch below fixes this for the drivers/xen tree.

I chose the general construct to store the respective return values in a
local var instead of an if (...) BUG(); because it looks like most of these
BUG_ON()'s need to be replaced by real error handling when going towards
production releases anyway, and then the local variable is going to be
needed anyway. (and besides the if construct tends to get unreable due to
the unlikely() etc).

Please apply.

Greetings,
   Arjan van de Ven