Fix crash formatting virtio console

Fix crash formatting virtio console

qemuBuildVirtioSerialPortDevStr was mistakenly accessing the
target.name field in the virDomainChrDef object for chardevs
belonging to a console. Those chardevs only have port set,
and if there's > 1 console, the > 1port number results in
trying to access a target.name with address 0x1

* src/qemu/qemu_command.c: Fix target.name handling and
  make code more robust wrt error reporting

* src/qemu/qemu_command.c: Conditionally access target.name