qemu-img create allows giving just a format and "-o help" to get a list
of the options supported by that format.  Users may not realize that the
protocol level may offer even more options, which they only get to see
by specifying a filename.

This patch adds a note to hint at that fact.

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