xen/apic: remove passing trapframe as argument

While otherwise a handy potential approach, getting the trapframe via the
argument isn't documented and isn't supposed to be used. While
ipi_bitmap_handler() and ipi_swi_handler() need to be passed the
trapframe as their arguments, the Xen functions can retrieve it from
curthread->td_intr_frame, which is the proper way.

Reviewed by: royger