qemu: handle -1 for pid in qemuDomainGetMachineName

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 eaf2c9f89.

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>