qemu: handle -1 for pid in qemuDomainGetMachineName

We call qemuDomainGetMachineName on domain start. On first
start (after daemon start) pid is 0 and virSystemdGetMachineNameByPID
don't get called. But after domain shutting down pid became -1 so
on next start virSystemdGetMachineNameByPID is called and returned an error.
Error is ignored so it is not critical. But at least on my system
(systemd-219 with extra patches) systemd-machined is crashed on
this request.

This behaviour is triggered by eaf2c9f8.
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_domain.c

@@ -9781,7 +9781,7 @@ qemuDomainGetMachineName(virDomainObjPtr vm)
     virQEMUDriverPtr driver = priv->driver;
     char *ret = NULL;
 
-    if (vm->pid) {
+    if (vm->pid > 0) {
         ret = virSystemdGetMachineNameByPID(vm->pid);
         if (!ret)
             virResetLastError();