blockdev: fix drive-mirror 'granularity' error message

Name the 'granularity' parameter and give its expected value range.
Previously the device name was mistakenly reported as the parameter
name.

Note that the error class is unchanged from ERROR_CLASS_GENERIC_ERROR.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>