target: Add DEF_TB_DEFAULT_ATTRIBS macro for virtual device attrs

This helper macro adds the default set of 30 device attributes for
virtual devices from existing target_core_configfs.c code, and moves
the definitions into a single macro to create the structs necessary
for backend drivers.

It allows them to populate their own external struct configfs_attribute
for se_subsystem_api->tb_cits.tb_dev_attrib_cit.ct_attrs.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>