xen/intr: add architecture interrupt counter initializer

[ehem_freebsd_m5p.com]

Summary

Unfortunately there isn't any architecture-independent interrupt counter
access or initialization. Due to this, add yet another hook to all the
architecture to setup the counter.

[ehem_freebsd_m5p.com]

I'm really unsure about this. There needs to be a solution to the issue of intrcnt_add() being x86-only, but I don't know whether the implementation in D31064 is appropriate (works as a stop-gap, but...).

[ehem_freebsd_m5p.com]

I expect to abandon as a better solution has shown up, I'm simply waiting for confirmation before doing so.

[ehem_freebsd_m5p.com]

Abandoning as a better approach was identified and has now passed the first testing round.