monitor: initialize global data from a constructor

Some monitor functions, most notably, monitor_cur() rely on global data being initialized by 'monitor_init_globals()'. The latter is called relatively late in startup. If code triggers error_report() before monitor_init_globals() is called, QEMU will abort when accessing the uninitialized monitor mutex. The critical monitor global data must be initialized from a constructor function, to improve the guarantee that it is done before any possible calls to monitor_cur(). Not only that, but the constructor must be marked to run before the default constructor in case any of them trigger error reporting. Note in particular that the RCU constructor will spawn a background thread so we might even have non-constructor QEMU code running concurrently with other constructors. As a general note, constructors should be extrememly careful about what QEMU code they invoke, as it cannot be guaranteed that the process is fully initialized and so not all normal QEMU API rules apply. Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org> Reviewed-by: Markus Armbruster <armbru@redhat.com> Fixes: e69ee454b5f9 (monitor: Make current monitor a per-coroutine property) Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>