xen/debug: remove usage of sbuf_{clear,finish}() on drained sbuf

Using sbuf_clear() on a drained sbuf is explicitly prohibited, and using
sbuf_finish() after printing a trace leads to a single trace being printed, as
after calling sbuf_finish() further attempts to use the same sbuf will lead to
a panic.

While there also switch to using xen_emergency_print() instead of attempting to
write directly to the hypervisor console. xen_emergency_print() can be
implemented per-arch to use a different mechanism than the console hypercall
(note the default implementation still uses the console hypercall).

Fixes: df62b8a25f47 ('xen: add a handler for the debug interrupt')
Sponsored by: Cloud Software Group
Reviewed by: markj
Differential review: https://reviews.freebsd.org/D45060