Currently we have three QemuOptsList (qemu_common_drive_opts,
qemu_legacy_drive_opts, and qemu_drive_opts), only qemu_drive_opts
is added to vm_config_groups[].

This patch changes query-command-line-options to access three local
QemuOptsLists for drive option, and merge the description items
together.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>