qga: Disable unsupported commands by default

Currently management softwares cannot know whether a qemu-ga command is
supported or not on the running platform until they actually execute it.
This patch disables unsupported commands at launch time of qemu-ga, so that
management softwares can check whether they are supported from 'enabled'
property of the result from 'guest-info' command.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>