add support for kern.intr.list aka intrctl(8) 'list' for xen

event_set_handler() and pirq_establish() now have extra intrname
parameter; shared intr_create_intrid() is used to provide the value

xen drivers were changed to pass the specific driver instance
name as the xname, e.g.  'vcpu0 clock' instead just 'clock', or
'xencons0' instead of 'xencons'

associated evcnt is now changed to use intrname - this matches native x86

(jdolecek)