COLO uses a worker context (iothread) to drive the chardev. All
backends are not able to switch the context, let's report an error in
this case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181205203737.9011-6-marcandre.lureau@redhat.com>
Reviewed-by: Li Zhijian <lizhiian@cn.fujitsu.com>
Reviewed-by: Zhang Chen <zhangckid@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>