qemu: monitor: Remove return value from qemuMonitorEmit* functions

qemu: monitor: Remove return value from qemuMonitorEmit* functions

The callers in the monitor code invoking the callbacks after events are
received don't actually check the return value from the callbacks and
there isn't really anything we could do on failure.

Remove the return value from the intermediary functions so we can later
remove them from the callback prototypes.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>