Creating an empty drive while specifying 'format' doesn't make sense.
The specified format driver would simply be ignored.

Make a set 'format' option an indication that a non-empty drive should
be created. This makes 'format' consistent with 'driver' and allows
using it with a block driver that doesn't need any other options (like
null-co/null-aio).
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>