block: Disable driver-specific options for 1.6

We don't want to commit to the API yet before everything is worked out.
Like already for 1.5, disable it again for the 1.6 release. This commit
is meant to be reverted after the 1.6 release.

The disabling of the driver-specific options is achieved by applying the
old checks while parsing the command line.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>