xen/control: print warning on call of xctrl_suspend()

Presently suspend/resume and migration aren't supported on Xen/ARM. As
such this shouldn't ever occur.

This likely applies to future Xen architectures (RISC-V) and
xctrl_suspend() needs dependency on intr_machdep.h fixed.

Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29599