triggered by recent xen3 hypervisor or tools): check sc->polling in
xencons_handler() and just return if set. Also, run xenconscn_getc() and putc()
at spltty().
While there, mark xencons_interface as volatile and use __UNVOLATILE() where
appropriate; and update the pointers in xencons_handler() if xenconscn_getc()
changed them (avoids replaying ddb commands in shell if ddb got called
by the magic string).
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