add kstat support for reading hardware counters.

this chip is annoyingly complicated, which is reflected in how
complicated it is to read counters off the chip. while we just use
ixl as a normal network interface, the chip is capable of being a
switch with physical ports, virtual ports and all sorts of other
functionality, and there are counters in different places for all
these different pieces. in our simple setup the driver interface
is mapped to a single physical port which we talk to via a single
virtual switch interface. this diff adds counters on each interface
for the physical port and for the virtual switch interface (vsi).

the port counters show what the hardware is doing, while the vsi
counters show how the driver is interacting with the chip. for
things like packet counters, these numbers tend to correlate strongly,
but there are some differences. if the chip drops packets cos there's
no descriptors on the rx ring, that's shown in the vsi counters.
problems talking to the physical network (eg, packet corruption off
the wire) are reported on the port counters.

on top of the chip just being complicated, reading the counters is
a complicated activity on its own. because the counters can be read
by multiple consumers in a virtualised environment, the counters
never really get reset. they are also stored in annoyingly small
fields. this means you basically have to poll the chip periodically
and calculate differences between the polls to avoid losing numbers
if they overflow too quickly.

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