xen/intr: partially revert "xen: fix IPI setup with EARLY_AP_STARTUP"

[ehem_freebsd_m5p.com]

Summary

Better to have a single use of device_get_nameunit() inside
xen_intr_bind_isrc(), than four outside of it. This is rather more
valuable since there is work in progress which requires the device be
available inside xen_intr_bind_isrc().

This partially reverts ca7af67ac958d3749ebfc4771cecf0cd7525e6a0.

[ehem_freebsd_m5p.com]

This needs checking, I'm mostly testing on ARM. I think what this does shouldn't recreate the breakage ca7af67ac958d3749ebfc4771cecf0cd7525e6a0 was trying to fix, but I'm not 100% certain.

Bigger issue now that I think about it is I may be propagating the wrong device_t here. Issue is this code is an interrupt controller and the ARM side needs a device_t for intr_isrc_register(), and that should be a device number from this code; not one propagated from an interrupt sink.

[ehem_freebsd_m5p.com]

I think this can be made unnecessary. Feels like this needs to wait for other pieces before any decisions are made.

[ehem_freebsd_m5p.com]

I've made builds which behaved appropriately without this commit. As such this appears unnecessary, so time to abandon it.