(FACT-755) set virtual to xen0 if proc/xen and dev/xen are not working although it ...

...is a xen

I had problems on some xen virtual hosts, that /dev/xen/* or /proc/xen was not present. But /dev/xvda1 (partition) was always present.
Looks good.  However, we prefer new code to use `&&` and `||` for logical expressions.  While not a problem in this instance, given the precedence of `and` and `or`, it can lead to unintended consequences in other expressions.  Unfortunately this was a standard enacted later in Facter/Puppet's development and we haven't scrubbed the code base of them; still, we'd prefer not introducing more into the code base.