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 <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

nertpinx/libvirt,eskultety/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,jardasgit/libvirt,eskultety/libvirt,libvirt/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,zippy2/libvirt,eskultety/libvirt,olafhering/libvirt,jfehlig/libvirt,jardasgit/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt