[libvirt] attach/detach device in json mode

[Wen Congyang]

Hi, all

The qemu does not support monitor_command drive_add and drive_del in json mode.
These two commands are important because attaching/detaching commands need them.

AFAIK, if we use libvirt provided by RHEL6(not upstream), we connect to qemu in
json mode. So, it is important to make qemu to support these monitor commands
in json mode.

Markus Armbruster(armbru at redhat.com) said these commands are designed badly, and
he has started to work on saner commands some time ago:
http://lists.nongnu.org/archive/html/qemu-devel/2011-02/msg02042.html

He suggested us to use human monitor passthrough in the meantime.
Should we modify libvirt to use human monitor passthrough command to support
attaching/detaching device before the saner commands are ready?