xen/intr: rename evtchn_cpu_(un)?mask_port() to xen_intr_*()

[ehem_freebsd_m5p.com]

Summary

Having these named evtchn_*() hints they are part of the core event
channel code, rather than part of the interrupt portion. This can
confuse when trying to do adjustments strictly effecting the interrupt
portion.

[ehem_freebsd_m5p.com]

This was split off of D30743. This one doesn't fit that well with the rest. This is also much lower value and not urgently needed.