systemd-detect-virt: detect s390 virtualization

A system that is running on a logical partition (LPAR) provided by
PR/SM has access to physical hardware (except CPU). It is true that
PR/SM abstracts the hardware, but only for sharing purposes.

Details are statet at:

http://publib.boulder.ibm.com/infocenter/eserver/v1r2/topic/eicaz/eicazzlpar.htm

-->--
In other words, PR/SM transforms physical resources into virtual resources so
that many logical partitions can share the same physical resources.
--<--

Still, from the OS point of view, the shared virtual resource is real
hardware. ConditionVirtualization must be set to false if the OS runs
directly on PR/SM (e.g. in an LPAR).

[zj: reorder code so that variables are not allocated when #if-def is
false. Add commit message.]