Clean up monitor_event to just use monitor_suspend/monitor_resume,
using mon->mux_out to protect against incorrect nesting (especially
on startup).

The only remaining case of reading suspend_cnt is in the can_read
callback, which is just advisory and can use qatomic_read.

As an extra benefit, mux_out is now simply protected by mon_lock.
Also, moving the prompt to the beginning of the main loop removes
it from the output in some error cases where QEMU does not actually
start successfully.  It is not a full fix and it would be nice to
also remove the monitor heading, but this is already a small (though
unintentional) improvement.

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