blockdev: Set 'format' indicates non-empty drive

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: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -148,12 +148,12 @@ class VM(object):
     def add_drive(self, path, opts='', interface='virtio'):
         '''Add a virtio-blk drive to the VM'''
         options = ['if=%s' % interface,
-                   'format=%s' % imgfmt,
                    'cache=%s' % cachemode,
                    'id=drive%d' % self._num_drives]
 
         if path is not None:
             options.append('file=%s' % path)
+            options.append('format=%s' % imgfmt)
 
         if opts:
             options.append(opts)