dict_table_get_nth_v_col(): Assert that no default value is ever set
on a virtual column. Virtual columns are not handled by the
instant ADD COLUMN mechanism.

ha_innobase_inplace_ctx::commit_instant(): Copy the default values
from instant_table to old_table.
Take deep copy of the dict_table_t::v_cols[].

instant_add_instant_try(): Copy the default values to instant_table.

dict_index_get_nth_field_def(): Define the function in the .h file.

dict_col_t::def_val.data: Change the data type to const void*
to reduce the number of type casts.