s390: disallow memory hotplug for the s390-virtio machine

That machine type doesn't currently support memory hotplug, so let's abort
if it is requested. Reason is, that the virtio queues are allocated for now
at the end of the initial ram - extending the ram is therefore not possible.
Reviewed-by: NMatthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/s390-virtio.c

@@ -23,6 +23,7 @@
 
 #include "hw/hw.h"
 #include "qapi/qmp/qerror.h"
+#include "qemu/error-report.h"
 #include "sysemu/block-backend.h"
 #include "sysemu/blockdev.h"
 #include "sysemu/sysemu.h"
@@ -268,6 +269,10 @@ static void s390_init(MachineState *machine)
     hwaddr virtio_region_len;
     hwaddr virtio_region_start;
 
+    if (machine->ram_slots) {
+        error_report("Memory hotplug not supported by the selected machine.");
+        exit(EXIT_FAILURE);
+    }
     /*
      * The storage increment size is a multiple of 1M and is a power of 2.
      * The number of storage increments must be MAX_STORAGE_INCREMENTS or