gdb: add function to check waiters

Useful to figure out which thread is waiting on what

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
trace: fix use of undefined symbol

You'll run into this when you have nested trace samples
with undefined backtrace elements.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>