s390x/virtio: use qemu_check_nic_model()

Switching to the generally used interface changes the output of

s390x-softmmu/qemu-system-s390x -net nic,model=?

from

S390 only supports VirtIO nics

to the rather more useful

qemu: Supported NIC models: virtio

while still giving us a sensible error message for unsupported
models:

s390x-softmmu/qemu-system-s390x -net nic,model=foo
qemu-system-s390x: Unsupported NIC model: foo

Acked-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/s390-virtio.c

diff --git a/hw/s390x/s390-virtio.c b/hw/s390x/s390-virtio.c
index 5dbb815..946325f 100644
--- a/hw/s390x/s390-virtio.c
+++ b/hw/s390x/s390-virtio.c
@@ -131,10 +131,7 @@
             nd->model = g_strdup("virtio");
         }
 
-        if (strcmp(nd->model, "virtio")) {
-            fprintf(stderr, "S390 only supports VirtIO nics\n");
-            exit(1);
-        }
+        qemu_check_nic_model(nd, "virtio");
 
         dev = qdev_create(bus, name);
         qdev_set_nic_properties(dev, nd);