block: Allow JSON filenames If the filename given to bdrv_open() is prefixed with "json:", parse the rest as a JSON object and merge the result into the options QDict. If there are conflicts, the options QDict takes precedence. Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>