qemu-img: Amendment support implies create_opts

qemu-img: Amendment support implies create_opts

Instead of checking whether a driver has a non-NULL create_opts we
should check whether it supports image amendment in the first place.  If
it does, it must have create_opts.

On the other hand, if it does not have create_opts (so it does not
support amendment either), the error message "does not support any
options" is a bit useless.  Stating clearly that the driver has no
amendment support whatsoever is probably better.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180509210023.20283-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>