gdb: fix 'osv thread' and 'osv thread apply all'

thread_list was populated with references to sched::thread* rather
than sched::thread. The commands were assuming the latter. As a result
the commands were printing and comparing not against sched::thread
address but against address of the pointer to sched::thread in
sched::thread_map.

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