qemu-img: Suppress unhelpful extra errors in convert, amend

img_convert() and img_amend() use qemu_opts_do_parse(), which reports
errors with qerror_report_err().  Its error messages aren't helpful
here, the caller reports one that actually makes sense.  Reproducer:

    $ qemu-img convert -o backing_format=raw in.img out.img
    qemu-img: Invalid parameter 'backing_format'
    qemu-img: Invalid options for file format 'raw'

To fix, propagate errors through qemu_opts_do_parse().  This lifts the
error reporting into callers.  Drop it from img_convert() and
img_amend(), keep it in qemu_chr_parse_compat(), bdrv_img_create().

Since I'm touching qemu_opts_do_parse() anyway, write a function
comment for it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
