xen: fix IPI setup with EARLY_AP_STARTUP

Current Xen IPI setup functions require that the caller provide a device in
order to obtain the name of the interrupt from it. With early AP startup this
device is no longer available at the point where IPIs are bound, and a KASSERT
would trigger:

panic: NULL pcpu device_t
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff82233a20
vpanic() at vpanic+0x186/frame 0xffffffff82233aa0
kassert_panic() at kassert_panic+0x126/frame 0xffffffff82233b10
xen_setup_cpus() at xen_setup_cpus+0x5b/frame 0xffffffff82233b50
mi_startup() at mi_startup+0x118/frame 0xffffffff82233b70
btext() at btext+0x2c

Fix this by no longer requiring the presence of a device in order to bind IPIs,
and simply use the "cpuX" format where X is the CPU identifier in order to
describe the interrupt.

Reported by: sbruno, cperciva
Tested by: sbruno
X-MFC-With: r310177
Sponsored by: Citrix Systems R&D