(FACT-755) Fix unprivileged Xen detection when /proc/xen is not present.

/dev/xvda1 exists on virtual hosts on physical xen hosts. Therefore use xenu as
virtual, so is_virtual is also true. Otherwise it is false.
So we can merge /proc/xen and /dev/xvda1
Merge pull request #812 from chiemseesurfer/master

(FACT-755) Fix unprivileged Xen detection when /proc/xen is not present.