virtio: virtconsole: Use seq_file for debugfs operations

Simplifies the code and is more conventional to what's used in the rest
of the kernel for debugfs ops.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Reviewed-by: Amit Shah <amit@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>