qemu: Avoid comparing size_t with -1

ncpus would be -1 on error and the cleanup for loop would not be skipped
in this case.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_capabilities.c

@@ -2965,7 +2965,7 @@ virQEMUCapsFetchCPUDefinitions(qemuMonitorPtr mon)
     size_t i;
 
     if ((ncpus = qemuMonitorGetCPUDefinitions(mon, &cpus)) < 0)
-        goto error;
+        return NULL;
 
     if (!(models = virDomainCapsCPUModelsNew(ncpus)))
         goto error;