target/s390x: use "qemu" cpu model in user mode

"any" does not exist, therefore resulting in a misleading error message.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170130145025.26475-1-david@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Cc: qemu-stable@nongnu.org
(cherry picked from commit d8923bc7)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>