s390x/cpumodel: store the CPU model in the CPU instance

A CPU model consists of a CPU definition, to which delta changes are
applied - features added or removed (e.g. z13-base,vx=on). In addition,
certain properties (e.g. cpu id) can later on change during migration
but belong into the CPU model. This data will later be filled from the
host model in the KVM case.

Therefore, store the configured CPU model inside the CPU instance, so
we can later on perform delta changes using properties.

For the "qemu" model, we emulate in TCG a z900. "host" will be
uninitialized (cpu->model == NULL) unless we have CPU model support in KVM
later on. The other models are all initialized from their definitions.
Only the "host" model can have a cpu->model == NULL.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-10-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
