block: Disable driver-specific options for 1.5

We don't want to commit to the API yet before everything is worked out.
Disable it for the 1.5 release. This commit is meant to be reverted
after the 1.5 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>