monitor: cleanup fetching of QMP requests

monitor: cleanup fetching of QMP requests

Use a continue statement so that "after going to sleep" is treated the same
way as "after processing a request".  Pull the monitor_lock critical
section out of monitor_qmp_requests_pop_any_with_lock() and protect
qmp_dispatcher_co_shutdown with the monitor_lock.

The two changes are complex to separate because monitor_qmp_dispatcher_co()
previously had a complicated logic to check for shutdown both before
and after going to sleep.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>