target/s390x/cpu_models: Make kvm_s390_get_host_cpu_model() return boolean

target/s390x/cpu_models: Make kvm_s390_get_host_cpu_model() return boolean

As error.h suggested, the best practice for callee is to return
something to indicate success / failure.

So make kvm_s390_get_host_cpu_model() return boolean and check the
returned boolean in get_max_cpu_model() instead of accessing @err.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240425031232.1586401-5-zhao1.liu@intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>