cpu: Bugfix for s390 CPU driver to return a host CPU model

cpu: Bugfix for s390 CPU driver to return a host CPU model

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: Daniel Hansel <daniel.hansel@linux.vnet.ibm.com>