The curent libvirt CPU driver for s390 does not return a host CPU model.
This patch returns 'host' according to the other platforms that would
not decode any CPU model.
This is an intermediate bugfix due to a discussion on OpenStack mailing
list. The final patch introducing the CPU model support for s390x will
exchange the hard-coded decode method.
Signed-off-by: NDaniel Hansel <daniel.hansel@linux.vnet.ibm.com>