qemu-char: Add qemu_chr_add_handlers_full() for GMaincontext

qemu-char: Add qemu_chr_add_handlers_full() for GMaincontext

Add qemu_chr_add_handlers_full() API, we can use
this API pass in a GMainContext,make handler run
in the context rather than main_loop.
This comments from Daniel P . Berrange.

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>