qemu-img convert: Deprecate using -n and -o together

bdrv_create options specified with -o have no effect when skipping image
creation with -n, so this doesn't make sense. Warn against the misuse
and deprecate the combination so we can make it a hard error later.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>