target/rd: Set ramdisk as non rotational device

Since a RAM backend device is not really a rotational device,
we set it as is_nonrot=1 which will be forwarded in VPD page 0xb1
(block device characteristics) response.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>