bdrv_img_create() takes an Error ** argument and uses it in the
conventional way, except for one place: when qemu_opts_do_parse()
fails, it first reports its error to stderr or the HMP monitor with
error_report_err(), then error_setg()'s a generic error.

When the caller reports that second error similarly, this produces two
consecutive error messages on stderr or the HMP monitor.

When the caller does something else with it, such as send it via QMP,
the first error still goes to stderr or the HMP monitor.  Fortunately,
no such caller exists.

Simply use the first error as is.  Update expected output of
qemu-iotest 049 accordingly.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-37-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>