trace: make trace points active state / backtrace individually controlled, with API calls for external (REST) management

In itself slightly underwelming, but sort of a prerequisite for ever adding
full REST control, with export etc. REST part obviously in other patch set
for the http server.

Signed-off-by: Calle Wilund <calle@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>