trace: fix tracepoint arguments that are strings in demand paged memory

If a char* tracepoint argument points into demand paged memory, we may
try to access it while it is unmapped, in a non-preemptible context.

Fix by handling the fault gracefully.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>