IPI_TRACE is not really supported on xen

x86 stack_save_td_running() can work safely only if IPI_TRACE is a
non-maskable interrupt. But at the moment FreeBSD/Xen does not provide
support for the NMI delivery mode. So, mark the functionality as
unsupported similarly to other platforms without NMI.
Maybe there is a way to provide a Xen-specific working
stack_save_td_running(), but I couldn't figure it out.

MFC after: 3 weeks
Sponsored by: Panzura