s390: fix short kernel command lines

The default kernel command line for s390 is
"root=/dev/ram0 ro"

When overriding this line, we have to ensure to also copy the \0 to
avoid false lines, for example, -append "root=/dev/vda" will result in
"root=/dev/vda0 ro" with the current code.
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/s390-virtio.c

@@ -230,7 +230,7 @@ static void s390_init(ram_addr_t my_ram_size,
 
     if (kernel_cmdline) {
         cpu_physical_memory_write(KERN_PARM_AREA, kernel_cmdline,
-                                  strlen(kernel_cmdline));
+                                  strlen(kernel_cmdline) + 1);
     }
 
     /* Create VirtIO network adapters */