The qemu-img.1 man page is missing the qed format from its list of
supported formats.  Document the image creation options for qed.
Suggested-by: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>