QMP: Fix error reporting in the async API

QMP: Fix error reporting in the async API

The current asynchronous command API doesn't return a QMP response
when the async command fails.

This is easy to reproduce with the balloon command (the sole async
command we have so far): run qemu w/o the '-balloon virtio' option
and try to issue the balloon command via QMP: no response will be
sent to the client.

This commit fixes the problem by making qmp_async_cmd_handler()
return the handler's error code and then calling
monitor_protocol_emitter() if the handler has returned an error.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>