blockdev: Move "file" to legacy_opts

Specifying the image filename through the "file" option is a legacy
option and should not be supported by blockdev-add (in that case, giving
a string for "file" references an existing block device).

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