target/user: Allow user to set block size before enabling device

The capability of setting hw_block_size was added along with 9c1cd1b6
"target/user: Only support full command pass-through", though default
setting override the user specified value during the enabling of device,
which called by target_configure_device() to set block_size matching
hw_block_size, result in user not able to set different block size other
than default 512.

This patch would use existing hw_block_size value if already set, otherwise
it would be set to default value(512).

Update: Fix the coding style issue.

(Drop unnecessary re-export of dev->dev_attrib.block_size - nab)

Signed-off-by: Sheng Yang <sheng@yasker.org>
Cc: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>