pci_vtcon_announce_port() sends VIRTIO_CONSOLE_PORT_NAME immediately
after VIRTIO_CONSOLE_DEVICE_ADD, without waiting for the guest to
initialize the port. The Windows virtio-serial driver ignores the
name if the port is not yet available, which can prevent the QEMU
guest agent from finding org.qemu.guest_agent.0.
Send the name after the guest reports successful port initialization
with VIRTIO_CONSOLE_PORT_READY, as QEMU does.
While here, validate the guest-supplied port ID before forming a
pointer into vsc_ports[].
This ports the PORT_NAME ordering fix from illumos change 18082.
Obtained from: illumos 643ee887be44404b690eeaff14f20a27ad3c715c