virtio: console: add port stats for bytes received, sent and discarded

This commit adds port-specific stats for the number of bytes received,
sent and discarded.  They're exposed via the debugfs interface.  This
data can be used to check for data loss bugs (or disprove such claims).
It can also be used for accounting, if there's such a need.

The stats remain valid throughout the lifetime of the port.  Unplugging
a port will reset the stats.  The numbers are not reset across port
opens/closes.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>