Update hotplug design doc
Update hotplug design doc

..to reflect the transition from HMP to QMP during hotplug
actions.

Signed-off-by: Dimitris Aragiorgis <4514117c4a99256e19437c7e3407c4ecbf98aec0@grnet.gr>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

onponomarev/ganeti,leshchevds/ganeti,bitemyapp/ganeti,mbakke/ganeti,bitemyapp/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,grnet/snf-ganeti,andir/ganeti,ganeti/ganeti,ganeti/ganeti,apyrgio/ganeti,andir/ganeti,mbakke/ganeti,dimara/ganeti,yiannist/ganeti,yiannist/ganeti,apyrgio/ganeti,onponomarev/ganeti,leshchevds/ganeti,dimara/ganeti