Libvirt has no way to probe if an option or property is supported,
This patch introduces a new qmp command to query command line
option information. hmp command isn't added because it's not needed.

Signed-off-by: Amos Kong <akong@redhat.com>
CC: Luiz Capitulino <lcapitulino@redhat.com>
CC: Osier Yang <jyang@redhat.com>
CC: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>