virtio_console: Introduce an ID allocator for virtual console numbers

virtio_console: Introduce an ID allocator for virtual console numbers

When a virtio console port is initialized, it is registered as an hvc
console using a virtual console number. If a KVM guest is started with
multiple virtio console devices, the same vtermno (or virtual console
number) can be used to allocate different hvc consoles, which leads to
various communication problems later on.

This is also reported in debugfs :

  # grep vtermno /sys/kernel/debug/virtio-ports/*
  /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1
  /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1
  /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2
  /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3

Replace the next_vtermno global with an ID allocator and start the
allocation at 1 as it is today. Also recycle IDs when a console port
is removed.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20221122134643.376184-1-clg@kaod.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>