S390: Enable -cpu help and QMP query-cpu-definitions

This enables qemu -cpu help to return a list of supported CPU models
on s390 and also to query for cpu definitions in the monitor.
Initially only cpu model = host is returned. This needs to be reworked
into a full-fledged CPU model handling later on.
This change is needed to allow libvirt exploiters (like OpenStack)
to specify a CPU model.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
[agraf: fix s390x-linux-user, adjust header locations]
Signed-off-by: Alexander Graf <agraf@suse.de>
