709: Change alignment of virtiofs structs to be based on the first field. r=mkroening a=joannejchen

Alignment of the virtiofs struct was previously based on the alignment of the largest field in fuse_in_header/fuse_out_header. Switched to explicitly align on u32, the type of the first field in the header.

Fixes #691

Co-authored-by: joannejchen <chenjjoanne@gmail.com>