tests/aio: Add support for larger max_buf_aio to vectored_big_iovcnt test

Fix a test failure that occurs when max_buf_aio exceeds 31, as the
calculated buffer size exceeds the 16 KB memory disk device. Increase
the MD device size to 1 MB to prevent truncation.

Rename MD_LEN to DEVICE_IO_LEN for clarity and add a more
descriptive error message.

Reviewed by: asomers
Approved by: asomers
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D52914