qemu: parse: Validate that the VM has at least one cpu

Libvirt's code relies on this fact so don't allow parsing a command line
which would have none.

Libvirtd would crash in the post parse callback on such config.

src/qemu/qemu_parse_command.c

@@ -1702,6 +1702,9 @@ qemuParseCommandLineSmp(virDomainDefPtr dom,
     if (maxcpus == 0)
         maxcpus = vcpus;
 
+    if (maxcpus == 0)
+        goto syntax;
+
     if (virDomainDefSetVcpusMax(dom, maxcpus, xmlopt) < 0)
         goto error;