doc/riscv: detail PV console input path

[baptleduc]

Summary

Expands the input section of the RISC-V PV console sequence diagram
(doc/riscv/xen-console.puml). The previous version modeled a keystroke as a
single user -> xenconsoled hop, which hid how a character actually travels
from the physical UART to the guest input ring.

Changes

• Add dom0 hvc0 tty and dom0 xl console participants to the diagram.
• Replace the single-step input hop with the full path:
  • keystroke into QEMU serial (Xen owns the physical UART)
  • serial_rx() → __serial_rx(), ring buffer write, VIRQ_CONSOLE
  • VIRQ_CONSOLE event channel notifies the hardware domain
  • HYPERVISOR_console_io(CONSOLEIO_read) drains serial_rx_ring
  • char surfaces on /dev/hvc0, read by xl console stdin
  • xl console writes char to the pty slave (/dev/pts/N)
  • xenconsoled poll() → handle_tty_read() → in[] ring
• Clarify that input uses handle_tty_read(), not handle_tty_write().
• Correct ring-write detail: xen_wmb() + explicit in_prod publish.

Testing

Render the diagram to confirm it is valid PlantUML:

plantuml doc/riscv/xen-console.puml

Inspect the generated image; verify the input sequence reads end-to-end with
no syntax errors.

PRs dependencies

Depends of #35 which should resolved zizmor fail. Must be merge after it and after the according rebase.

Checklist

• Check the compiled diagram
• Reviewed internally by @olkur

[baptleduc]

As mentioned here #36, zizmor fail will be resolved in #35. So this PR should be merge after #35 and after the appropriate rebase.

[olkur]

Changes itself LGTM